Male and female rats were raised from weaning either in isolation or in a large colony. At 65 days of age, half the rats in each environment were moved to the other. At 80 days, the animals were given continuous access to water and to a sequence of 7 solutions: 3 sweet or bitter-sweet control solutions and 4 different concentrations of morphine hydrochloride (MHCl) in 10% sucrose solution. Rats housed in the colony at the time of testing drank less MHCl solution than isolated rats, but no less of the control solutions. Colony-dwelling rats previously housed in isolation tended to drink more MHCl solution than those housed in the colony since weaning, but this effect reached statistical significance only at the lowest concentration of MHCl. These data were related to the hypothesis that colony rats avoid morphine because it interferes with complex, species-specific behavior.