1. 4-Ethynylbiphenyl undergoes extensive metabolism in the rat and the rabbit, involving aromatic hydroxylation and oxidation of the ethynyl group. No metabolites containing the intact ethynyl group were detected. 2. In the rat unchanged 4-ethynylbiphenyl was concentrated initially in the adipose tissue. No other tissues accumulated significant amounts of radioactivity. 3. The major metabolites were the same in both the rat and the rabbit, namely 4'-hydroxybiphenyl-4-ylacetic acid (90-95% of dose) and biphenyl-4-ylacetic acid (2-10% of dose). 4. Excretion was slower in the rat than in the rabbit, probably because of greater biliary and faecal excretion in the rat. Biliary excretion and enterohepatic circulation of biphenyl-4-ylacetic acid and 4'-hydroxybiphenyl-4-ylacetic acid were demonstrated in the rat.