Signal detection analysis was used to examine the effects of d-amphetamine and of morphine on delayed visual discrimination (delay intervals: 0-4-8-16 s) in male rats. The probability of response repetition in the discrete trial two-choice discrimination procedure was used as an additional behavioral measure. d-Amphetamine (0.16-0.33 mg/kg) decreased SI ( a measure of the animals' sensitivity to the discriminative stimuli) at delays between stimulus presentation and opportunity for responding of 4-16 s, and did not affect SI at the 0 s delay. Morphine (1-3 mg/kg) decreased SI at all delay conditions. d-Amphetamine, but not morphine, affected RI (a measure of the animals' bias towards responding on one lever or the other) and increased the probability of response repetition. The bias measure B inches was affected neither by d-amphetamine nor by morphine. It is concluded that d-amphetamine, but not morphine, produces a deterioration of delayed discrimination performance, probably as a result of drug-induced response perseveration. It is suggested that, under the conditions of the present study, the selective deterioration of discrimination performance after d-amphetamine at delays which are longer than 0 s may not be primarily related to a drug-induced disruption of a short-term memory mechanism, but may be related to drug effects on response output.