The US Supreme Court has begun hearing its first case under new leadership, concerning the emotionally charged issue of assisted suicide.
The case will be an early test of Mr Roberts' approach
New Chief Justice John Roberts is presiding over his first case since being sworn in last month.
He appeared sceptical of claims by the goverment of Oregon that US state have the final say on medical issues.
The US government wants to overturn an Oregon law allowing terminally ill patients to end their own lives.
The court's ruling will have implications across the US. Currently, Oregon is the only state to allow assisted suicide, but others hope to follow suit.
The case will also be closely watched for signs of the court's direction under Mr Roberts.
Mr Roberts was appointed to replace the late William Rehnquist, who died after suffering cancer.
State v country
Mr Roberts is Roman Catholic and considered a conservative, and so might be expected to vote against euthanasia.
However, he is also seen as an advocate of a state's right to rule its own affairs, against interference from federal government.
As the case opened on Wednesday, Mr Roberts put tough questions to Oregon's lawyers, asking whether such state decisions would undermine the effectiveness and uniformity of federal law.
Other justices seemed split in their positions, with one referring to states "gutting" the federal law and another saying regulation of assisted suicide was a "bizarre" extrapolation of laws aimed at drug abuse.
Oregon has allowed euthanasia since passing a law in a 1997 state referendum.
Since then 208 people, mostly cancer patients, have elected to die under the law, which has strict conditions.
Patients must be in final six months of terminal illness
Patients must make two oral requests and one written request to die, separated by a two-week period
Patients must be mentally competent to make decision
Two doctors must confirm diagnosis
Lethal prescription of drugs prescribed by doctor and administered by patients themselves
A patient must have less than six months to live, must be deemed by two doctors as mentally fit to take the decision and must present one written and two oral demands over a certain period.
He can then obtain lethal medication from his doctor but must administer it himself.
The Bush administration has had the Oregon law in its sights for some years.
Former Attorney General John Ashcroft tried to annul the law in 2002, saying it depended on an improper use of medication by doctors and violated federal drug laws.
His order was overruled by an appeals court.
If the Supreme Court rules that the Oregon law is constitutional, several states are poised to introduce similar legislation of their own.
The court itself has personal experience of terminal illness.
Rehnquist died of an untreatable cancer.
Three of the nine justices have had cancer and a fourth has a wife who counsels dying young cancer patients.
Meanwhile Justice Sandra Day O'Connor, a former breast cancer patient, is due to step down from the court when Congress confirms her replacement.
Ms O'Connor has often had the swing vote on contentious court rulings. She provided the key fifth vote in a 1997 decision that patients have no constitutional right to doctor-assisted suicide, but that allowed individual states room to experiment.
If she steps down before the case is completed, her vote will not count.
In the event of a 4-4 tie, the court will probably order the case to be heard again.
President George Bush has nominated White House lawyer Harriet Miers to replace Ms Day O'Connor.