1. [3H]N-[1-(2-Benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) labels in vivo the dopamine uptake complex in the mouse striatum. 2. In mice treated with gamma-butyrolactone (GBL, 400 mg/kg), [3H]BTCP specific binding was increased and ID50 values of BTCP and cocaine for the prevention of [3H]BTCP binding were significantly lowered. 3. In GBL-treated mice, cocaine (5 mg/kg, s.c.) had no effect on the BTCP ID50 value, whereas GBR 12783 (2 mg/kg, s.c.) increased it significantly. 4. Thus in vivo, endogenous dopamine and cocaine are competitive and non-competitive inhibitors, respectively, of the binding of [3H]BTCP.