A renin-angiotensin system, separate to that in the periphery, has been found in the brain. Angiotensin-converting enzyme (ACE) is crucial in the synthesis of angiotensin II, breakdown of bradykinin and the hydrolysis of several other neuropeptides such as enkephalin, substance P, dynorphin and neurotensin. Changes in the levels of ACE have been found in brains of schizophrenia patients, suggesting an involvement of ACE in the illness which awaits further investigation. Prepulse inhibition (PPI) has been suggested to be an operational measure of sensorimotor gating and is disrupted in patients with schizophrenia. We found that ACE knockout mice have increased startle responses but no differences in baseline PPI compared to wildtype controls. Treatment with the dopamine receptor agonist, apomorphine, or the dopamine-releasing drug, amphetamine, produced significant disruption of PPI in control mice but not in ACE knockout mice. Pretreatment with the ACE inhibitor, captopril, which itself did not affect PPI, caused a reduction in the effect of apomorphine on PPI, similar to that seen in the ACE knockout mice. These data suggest an important role of ACE substrates in modulating dopaminergic mechanisms involved in PPI. Further studies are needed to ascertain if angiotensin or other neuropeptides are involved in these interactions and to investigate the neurochemical mechanism behind these effects.