Rats implanted with electrodes for polygraphic recording were administered with L-PIA (0.115 mg/kg, i.p.), caffeine (15 mg/kg, i.p.) or L-PIA (0.115 mg/kg, i.p.) + caffeine (15 mg/kg, i.p.) and recorded for 6 h. The results show that administration of L-PIA increased S 2 by 54 min suggesting that stimulation of adenosine receptors promotes deep sleep. Administration of L-PIA failed to produce the same effect in the presence of caffeine, a finding consistent with the hypothesis that the CNS stimulant effect of caffeine and other methylxanthines is due to their ability to antagonize depressant effects of endogenous adenosine.