A New York court is set to decide whether chimpanzees are entitled to "legal personhood" and the protections that go with it.
The case, which experts say is the first of its kind, revolves around Tommy the chimp, who is currently being kept by his owner in a cage in upstate New York.
Lawyer Steven Wise represents an animal rights group called the "Nonhuman Rights Project" and has been campaigning for decades to extend human rights to other intelligent animals.
Its website states: "Our mission is to change the common law status of at least some nonhuman animals from mere 'things', which lack the capacity to possess any legal right, to 'persons', who possess such fundamental rights as bodily integrity and bodily liberty, and those other legal rights to which evolving standards of morality, scientific discovery, and human experience entitle them."
Mr Wise is seeking a ruling that 26-year-old Tommy has been unlawfully imprisoned and should be cared for in a Florida sanctuary.
Previous lawsuits have been thrown out, but Mr Wise has appealed against the decision. If the New York state appeals court rules in his favour it could strengthen the case for extended rights for other high intelligence animals such as dolphins and elephants.
The Boston-based lawyer has also lodged an appeal on behalf of another chimp called Kiko. He is using a legal argument usually deployed by prisoners who feel they have been locked up illegally.
Tommy's owner, Patrick Laverty, has waived his right to make an argument before the court, but has said previously that Tommy actually lives in a state-of-the-art facility and that he has been on a waiting list for an animal sanctuary for some time.
He told the Albany Times Union newspaper: "He's got a lot of enrichment. He's got colour TV, cable and a stereo � he likes being by himself."
Some legal experts such as US Circuit Judge Richard Posner and New York University law professor Richard Epstein have criticised the court case.
According to Reuters news agency, both men believe that if animals are afforded the rights traditionally reserved for humans, courts could be overwhelmed by difficult legal questions.
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