The serum concentrations of unchanged and glucuronized drug were determined after intravenous injection of pentazocine (PA) in rabbits. A compartment model isolating the liver from the central compartment was proposed to explain the observed time courses of the concentration, and the resulting convolution equation was M (t)=∫¹₀D (t-θ) G (θ) dθ, where M (t) and D (t) are the serum concentrations of glucuronized PA and unchanged PA after intravenous injection, respectively. The weight function, G (t), which represents the response for a pulse input of glucuronized PA was estimated to be G (t)=0.00933 (e^-0.00818t-e^-0.223t) (min^-1). The simultaneous intravenous administration of salicylamide (SAM) had no effect on the glucuronization of PA in serum. The serum concentration of glucuronized PA calculated by numerical convolution of G (t) and the published serum concentration of unchanged PA after oral administration represents that of glucuronide produced from the unchanged PA which escaped the first-pass effect. This calculated value was far below the published serum concentration of glucuronized PA after oral administration, and the difference represents the glucuronized PA produced by the first-pass effect. The area under the concentration-versus-time curve of glucuronized PA produced by the first-pass effect was 274 times larger than that of glucuronized PA produced from unchanged PA in the serum. Simultaneous oral administration of SAM only inhibits the first-pass effect.