Two D 2 dopamine (DA) receptor antagonists, haloperidol and L-sulpiride, have been tested for their ability to modify the latency to sleep. These drugs were given acutely prior to the end of the sleep deprivation period, in time to obtain peak action during the testing session. Haloperidol (0.2 mg/kg) was highly effective and potent in reducing latency to sleep, while L-sulpiride was much weaker (