Postsynaptic inhibitory gamma-aminobutyric acid-A (GABAA)-receptor-mediated current responses were measured using simultaneous pre- and postsynaptic whole cell recordings in primary cell cultures of rat striatum. Substitution of Sr2+ for extracellular Ca2+ strongly desynchronized the inhibitory postsynaptic currents (IPSCs), resulting in a succession of asynchronous IPSCs (asIPSCs). The rise times and decay time constants of individual evoked asIPSCs were not significantly different from those of miniature IPSCs that are the result of spontaneous vesicular release of GABA. Thus asIPSCs reflect quantal transmission at the individual contacts made by one presynaptic neuron on the recorded postsynaptic cell. Increasing the concentration of Sr2+ from 2 to 10 mM and decreasing that of Mg2+ from 5 to 1 mM produced an increase in the frequency of asIPSCs consistent with an enhancement of the mean probability of release (Pr). At the same time the amplitude distribution of asIPSCs was shifted toward larger values, whereas responses to exogenously applied GABA on average were slightly decreased in amplitude. Application of the GABAB-receptor agonist baclofen (3-10 microM) strongly reduced the frequency of asIPSC, consistent with a decrease in Pr, and led to a shift of the amplitude distribution toward smaller values. Baclofen had no effect on responses to exogenously applied GABA. In summary, our data suggest that at striatal inhibitory connections the weight of single contacts may be controlled presynaptically by variation in the amount of transmitter released.