Last year, the nation's highest court said that using marijuana to relieve pain violated the federal Controlled Substances Act. This week, the court said the same federal law could not be used to punish doctors who prescribe a lethal cocktail for terminally ill patients in Oregon.

While the results might seem contradictory, taken together the rulings firmly establish the central purpose of the Controlled Substances Act: to combat illicit drug use and trafficking.

"Marijuana was exactly the kind of thing that is at the heart of the Controlled Substances Act. Congress really wanted to seriously regulate this particular drug in all of its forms," said Robert L. Tsai, an assistant professor at the University of Oregon School of Law. "By contrast, it's hard to say -- and finally the court rejected the argument -- that the dispensing of certain drugs to ease one's death really goes to the heart of the Controlled Substances Act."

More broadly, the two rulings referee a swirling power struggle among Congress, the executive branch and the states.

In the marijuana case, the court said Congress' authority to regulate marijuana trumped laws in California and 10 other states that allow the drug's use for medical purposes.

In the assisted suicide case, by contrast, the court said the federal executive -- the U.S. attorney general -- exceeded the power Congress had given him under the law.

And that ruling, say several experts, could suggest how the court is leaning in other major executive-power cases coming down the pike, including the handling of enemy combatants and warrantless surveillance by the National Security Agency.

"It gets kind of dry and technical," Willamette University law professor Valerie J. Vollmar said about the assisted suicide case. "But I think the most interesting thing about it is this executive-power struggle that is going on."

"The majority is saying, `Wait a minute, there are limits to what you, the attorney general, can do on behalf of the executive branch,"' Vollmar said. "The dissenters are saying you can do almost anything. That's a pretty extreme position. And I think it's of concern that Chief Justice Roberts signed onto the dissent."

The two cases certainly have similarities: Both involved state ballot initiatives that allow the use of federally regulated drugs to treat pain and terminal illness. While multiple states have approved the medical use of marijuana, no states beside Oregon have adopted an assisted-suicide law.

The key difference between the two cases: Congress explicitly outlawed any use of marijuana while the drugs prescribed to terminally ill patients -- secobarbital and pentobarbital -- have legal uses.

The marijuana case came out of California, where state lawyers argued that Congress had exceeded its authority to regulate interstate commerce when it banned locally grown marijuana used for medical purposes.

The court rejected the argument 6-3, with Sandra Day O'Connor, then-Chief Justice William Rehnquist and Clarence Thomas in dissent.

Oregon won its case by the same vote, but most of the justices switched sides.

And instead of hinging on the big-sweep question of Congress' constitutional authority, the Oregon case came down to the arcane rules of statutory construction and deference. In simple terms, if the Controlled Substances Act doesn't mention assisted suicide, should the courts defer to the attorney general's determination that it is not a legitimate medical use of drugs to help people die.

"It's an administrative law geek's idea of heaven," said Orin S. Kerr, an associate professor at George Washington University Law School in Washington, D.C. "It's a very technical decision."

And unlike the marijuana ruling, it does not end the debate.

Nothing about Tuesday's ruling prohibits Congress from clearly giving the attorney general authority to prosecute doctors who prescribe drugs for assisted suicide.

But if Congress outlaws assisted suicide, supporters expect to go back to court and point out other differences between the marijuana and assisted-suicide laws.

The most obvious: Congress may be able to ban a substance such as marijuana, but is it beyond its authority to intrude on the traditional state determination of what is the legitimate practice of medicine?

"If we ever are faced with that battle," said Mary H. Williams, Oregon's solicitor general, "I think there are a number of arguments."