The effect of hydrochlorothiazide on the pharmacokinetics of carteolol in rabbits was studied. Animals were divided into two groups (group I and group II). Group I received carteolol (12mg/kg) and group II received carteolol (12mg/kg) with hydrochlorothiazide (20mg/kg) orally. The carteolol concentration in serum was measured by spectrofluorometric method and its pharmacokinetic parameter values were calculated. The serum concentration of carteolol in group II was significantly increased when compared with those in group I at 10min (p<0.01), and at 30min (p<0.05) after p. o. administration. In addition, the absorption rate constant of carteolol in group II was slightly increased and Tmax of carteolol in group II was significantly shortened (p<0.05) and Cmax of carteolol in group II was significantly increased (p<0.02) when compared with those in group I. But elimination rate constant and biological half-life of carteolol were similar in both groups.