Rats were exposed to a conditioning procedure that varied the duration of overlap between a light–noise conditioned stimulus (CS) and the effects of a morphine (5 mg/kg) unconditioned stimulus (US). Three paired (P) groups differed in CS duration (5, 15, or 60 min) but had the same CS–US interval (30 s). A control group (U) received explicitly unpaired presentations of CS and US. P groups showed CS-specific attenuation of the bradycardic response and enhancement of the hyperthermic response to morphine. During placebo tests, the CS elicited conditioned increases in heart rate and body temperature in Groups P15 and P60. Group P5 showed a conditioned increase in heart rate but not in body temperature. Overall, strength of conditioning was directly related to CS duration. These data indicate that duration of overlap between a CS and drug-induced changes in a target response system is an important determinant of Pavlovian drug conditioning.