In adult, sexually-experienced male rats, the intracerebroventricular injection of NPY caused a dose-related inhibition of copulatory behaviour, all parameters (mount, intromission and ejaculation latencies, mount and intromission frequencies, mean inter-intromission interval, post-ejaculatory interval) being significantly worsened at the dose of 8 micrograms/rat. Since rats were deprived of food during the behavioural test, it is concluded that inhibition of sexual behaviour is a 'true', direct behavioural effect of NPY, not due to a shift towards increased feeding.