At weaning, rats were housed either individually or in pairs and as adults were trained to poke their nose in and out of a port that dispensed a 2-s exposure of sufentanil aerosol (50-micrograms/ml solution). During the acquisition phase, which consisted of five nightly sessions lasting 14-16 h, individually caged rats responded for more sufentanil aerosol than did pair-caged animals when the fixed ratio (FR) requirement was gradually increased from FR 1 to FR 5 over the five sessions. During the maintenance phase, which consisted of daytime 2-h sessions at an FR 5 schedule of reinforcement, there were no differences between individually and pair-caged animals responding for sufentanil or for water vapor. Both groups responded significantly more for sufentanil than for water vapor. Based upon present evidence, it is suggested that environmental and biologic determinants may change psychomotor behavior in a way that could influence the rate by which animals acquire drug-seeking behavior.