Neuropeptides that may induce behavioral activation--thyrotropin-releasing hormone (TRH), oxytocin (OXY), and prolactin (PRL)--were tested on thiopenthal-induced narcosis after IV administration in male rats. TRH caused a significant shortening of sleeping time at the doses of 3 and 5 mg/kg, but did not change this parameter at lower doses. Oxytocin was effective at all doses tested (200, 300, and 400 micrograms/kg). Prolactin also shortened sleeping time at the doses of 0.2 and 1 mg/kg administered IV, slightly increasing it at the dose of 5 mg/kg. These results indicate that various neuropeptides are capable of reducing the duration of thiopenthal-induced sleep in rats.