Ciprofibrate, a new orally effective hypolipidemic agent like clofibrate, suppressed tyloxapol-induced hypercholesterolemia in rats. Ciprofibrate at 10 mg/kg was effective. Clofibrate required a dosage of 180 mg/kg to suppress the tyloxapol effect. Norepinephrine-induced free fatty acid release was inhibited by clofibrate in rats in accordance with earlier findings. Ciprofibrate and lifibrate differed from clofibrate in that, at hypocholesterolemically effective doses, neither inhibited the hormone sensitive lipase in vivo.