Earlier studies had indicated the possibility of the involvement of the alpha(1) adrenergic receptors on the medial preoptic (mPOA) neurons in hypnogenesis. Microinjection of alpha1 agonist methoxamine (0.05 microg) into the mPOA of the rats, maintained at an ambient temperature (Tamb) of 24 degrees C, did not produce any significant change in sleep-wakefulness (S-W), except for an arousal of short duration, which coincided with the steep fall in body temperature (Tb). It was probably not possible to elicit sleep, because of hypothermia simultaneously produced by this drug. To test this hypothesis, experiments were conducted in an environment where the effects of hypothermia could be partially compensated for. When the rats were allowed to select their Tamb, they moved over to a compartment having a higher Tamb of 30 degrees C, on administration of methoxamine into the mPOA. Simultaneously, there was induction of sleep, which lasted for 60 min. Thus, it was possible to demonstrate the hypnogenic action of methoxamine, which was masked by hypothermic action of the drug. The findings indicate that the alpha1 adrenergic receptors in the mPOA are involved in the induction of sleep.