Etofibrate (Lipo-Merz) is a newer antilipaemic agent that contains the nicotinic and clofibric acid moieties joined together through a diester link with ethylene glycol. The bioavailability of these moieties in etofibrate was compared to that from equimolar amounts of these drugs administered alone to rhesus monkeys (clofibric acid 354 mg, nicotinic acid 203 mg). For this comparative purpose, analytical methods were chosen which converted etofibrate and its initial metabolites into clofibric and nicotinic acid. Following this treatment, the rate and extent of bioavailability of unchanged nicotinic acid from etofibrate was significantly lower (P less than 0.01) than that from nicotinic acid alone. Mean peak whole blood levels of 11.9 micrograms/ml at 2.8 h and 35.3 micrograms/ml at 1.3 h, respectively, occurred after administration of etofibrate and nicotinic acid alone. The extent of bioavailability of clofibric acid hydrolysed from etofibrate was not significantly different (P greater than 0.05) from that from clofibric acid alone. However, mean peak plasma levels of 127.3 micrograms/ml at 3.6 h and 170.8 micrograms/ml at 2.4 h, respectively, occurred after administration of etofibrate and clofibric acid alone and were significantly different (P less than 0.01). The rates and extent of bioavailability of nicotinic acid or clofibric acid administered as a mixture were similar to that from these drugs administered alone. Thus either drug did not affect the absorption of the other in rhesus monkeys. The half-lives of nicotinic acid (in whole blood) and clofibric acid (in plasma) in rhesus monkeys when these drugs were administered alone were 9.9 h and 1.8 h, respectively.