The uptake of [3H]dopamine (DA) into rat striatal synaptosomes in the presence of a monoamine oxidase inhibitor was studied using a filtration technique. After a 10-min preincubation period, a fast initial uptake of [3H]DA was seen. Uptake reached a maximum after 4 min of incubation. If incubation was continued for more than 7 min, a gradual decrease in synaptosomal [3H]DA levels was found. Uptake was dependent on preincubation time; initial uptake velocity and maximal uptake decreased irreversibly with increasing preincubation periods. Moreover, the capacity of the synaptosomes to retain the [3H]DA during longer incubation times was progressively affected. The decrease in initial uptake activity was due to a decrease in the Vmax of the transport system. Dithiothreitol (2.8 mM) protected synaptosomal uptake activity against deterioration at 37 degrees C. Also, DA itself (10(-7)M) stabilized the uptake mechanism if added to the suspension before preincubation was started. Since [3H]DA uptake observed after loading the synaptosomes with labeled DA was similar to the uptake seen if the synaptosomes were not previously loaded with DA, it was concluded that under these conditions synaptosomal DA is completely exchangeable with exogenous substrate. Prolonged storage of the synaptosomes at 0 degree C also resulted in a time-dependent decrease in uptake activity (t1/2 = 116 min). The addition of unlabeled DA or dithiothreitol to the suspension did not affect instability at 0 degree C.