It has been shown that the acoustic startle reflex is inhibited by weak auditory prepulses presented 30-500 ms prior to the startling stimulus and that this prepulse inhibition (PPI) is modulated by ventral striato-pallidal circuitry. However, dorsal striatal modulation of PPI has not been examined. Cell-specific lesions and intracerebral drug infusions were used to elucidate striatal modulation of PPI. Quinolinic acid lesions of the ventral and caudodorsal striatum significantly decreased PPI, whereas lesions of the rostrodorsal and middorsal striatum did not significantly alter PPI. Infusion of the GABA-A antagonist picrotoxin into the ventral and caudal dorsal pallidum also significantly reduced PPI, whereas rostral pallidal picrotoxin infusion had no significant effect. Thus, PPI in the rat seems to be modulated by both ventral and caudodorsal striato-pallidal circuitry, but not by rostrodorsal or middorsal striato-pallidal projections.