Prepulse inhibition (PPI) is the inhibition of an acoustic startle response (ASR) that is observed when a weak prepulse is presented shortly before a startling stimulus. Here we studied in Wistar rats the dependence of PPI on variations of the interstimulus interval (ISI; from 25-1020ms) after treatment with various drugs that are known to disrupt PPI. The motor response to the prepulse itself (prepulse elicited reaction, PER) was also studied. The direct dopamine receptor agonist apomorphine, the non-competitive NMDA glutamate receptor antagonist MK-801, and the cannabinoid CB1 receptor agonist WIN 55,212-2 all reduced PPI, depending on the ISI, with different effects on the PER and/or pulse alone. The serotonin 2A receptor agonist DOI tended to reduce PPI. The cannabinoid CB1 receptor antagonist AM 251 did neither affect PPI nor the responses to prepulses or startling noise pulses. Taken together this study supports the current notion of a pharmacologically complex pattern of regulation of PPI at different ISIs and suggests that the PER is a miniature ASR that does, however, not predict the level of PPI.