Absorption, distribution, metabolism and excretion were investigated after a single oral administration of 14C-fadrozole hydrochloride (14C-CGS 16949A) to rats, mainly females. 1. The plasma levels of radioactivity in non-fasted female rats reached the peak (Cmax) of 379ng eq. of CGS 16949 (the base of CGS 16949A)/ml at 2hr after administration and then declined with a half-life of 6.8hr from 4hr to 24hr. The AUC was similar to that after intravenous administration. The plasma concentration in non-fasted male rats reached the Cmax of 295ng eq. of CGS 16949/ml at 2hr and then declined with a half-life of 8.2hr from 4hr to 24hr. While the AUC were similar in non-fasted and fasted female rats, the Cmax was 1.3 times higher in fasted females. The Cmax and AUC increased in dose dependent manner at the doses of 0.2, 1 and 5mg/kg. The plasma concentration of unchanged CGS 16949 reached the Cmax of 269ng eq. of CGS 16949/ml at 30min and then declined with a half-life of 3.1hr from 4hr to 12hr after a single oral administration of 14C-CGS 16949A 1mg/kg to non-fasted female rats. 2. In non-fasted female rats, 89.3 and 10.2% of the dose was excerted within 168hours in the urine and feces, respectively. In the bile an urine of bile-duct cannulated female rats, 24.9 and 59.4% of the dose, respectively, were excreted within 48hr. The intraduodenal administration of biliary radioactive materials resulted in excretion within 48hours of 31.0 and 56.9% to the bile and urine, respectively. 3. The majority of tissues in female rats showed a maximal levels of radioactivity 2hr after administration. At the maximum, adrenal gland, stomach and liver exhibited high radioactivity. Radioactivity of all the tissues decreased to less than 5% of the maximum 168hr after administration. 4. In rat plasma, the unchanged compound (CGS 16949), trans-8-hydroxy metabolite (CGP 45383), cis-8-hydroxy metabolite (CGP 45384) and 8-oxo metabolite (CGP 45385) were mainly found, and the metabolite with hydantoin-type structure (MP2) accounted for the major fraction of radioactivity 24hr after administration. The relative amount of CGS 16949, CGP 45383 and CGP 45384 accounted for 18.1, 34.8 and 25.0% of urinary radioactivity, respectively, and the major metabolite in bile would be the glucuronide of CGP 45383.