The discriminative stimulus effects of a low dose of sodium pentobarbital (4 mg/kg, sc) were investigated in 10 rats trained to discriminate between the drug and saline under a 2-lever choice procedure. A food pellet was delivered after every 10 depressions of the appropriate lever, which lever was appropriate being determined by whether the drug or saline was administered prior to the daily session. Discrimination of the substances was said to have been attained when more than 8 out of the 1st 10 responses were to the appropriate lever for 10 consecutive days, and 7 rats met the criterion after 43 to 96 sessions. Stimulus generalization tests revealed the stimulus properties of the drug at 4 mg/kg to be similar to those of diazepam 0.25 mg/kg, sc and ethanol 1 g/kg, ig, but not of various doses of d-amphetamine, morphine, pentazocine, chlorpromazine, and imipramine. It was thus found that the rats could discriminate a relatively low dose of pentobarbital and that its stimulus effects do not differ from those at higher doses reported by other investigators.