In vivo brain microdialysis was used to monitor extracellular levels of dopamine (DA) in the nucleus accumbens (NAc) of rats during exposure to startling acoustic stimuli. Ten rats were prepared with guide cannulae into which dialysis probes were inserted 1 d before testing. Two to three hours after the start of perfusion, rats were placed into the startle chamber and exposed to a continuous 70 dB(A) background noise. Dialysis samples (2.0 microliter/min) were collected at 6 min intervals. Startle pulses (120 dB[A] noise) were presented in 20-trial blocks lasting 5 min. In some blocks, an 86 dB[A] prepulse preceded each of the 20 pulses by 100 msec, with the order of presentation of pulse-alone or prepulse + pulse blocks being counterbalanced between animals. Three to six sample periods intervened between stimulus- containing blocks. Monoamine and metabolite levels were measured using HPLC with electro-chemical detection. During the presentations of startling stimuli, DA levels in the NAc decreased relative to the immediately preceding 12 min baseline. This decrease in DA was maintained for only one additional sample period. By contrast, the presentation of prepulse + pulse trials failed to affect dialysate levels of NAc DA during or immediately after the stimulation. Thus, startling acoustic stimuli produce significant and transient decreases in dialysate levels of DA in the NAc. Furthermore, prepulse stimuli effectively inhibit these neurochemical effects of starting stimuli, in parallel with their establish ability to inhibit the amplitude the behavioral startle response.