The

influences of the physical and social aspects of the environment on temperament in rats are reported in two experiments. In Experiment One changes in animal behavior due to social isolation and enrichment of the physical environment were studied in rats subjected to experimental conditions when they were 25-30 days old. Enrichment of the physical environment led to decreased need for light stimulation and increased exploratory behavior. Social isolation led to an increase in emotional reactivity. Experiment Twowas analogous to Experiment One, diff"ering only in that 60-70 day-old animals were given the experimental conditions. In this case enriched physical environment and social isolation led to decreased exploratory behavior. Need for light stimulation was aff'ected by the interaction of experimental variables. Need for kinesthetic and tactile stimulation were affected by the social environment. Enrichment of the physical environment also led to a decrease in emotional reactivity in the rats. Our experiments indicate that environmental efl'ects were age-contingent. It is proposed that there is probably a relationship between categories of environment and temperament. Further, our findings draw attention to the possibility of a related evolution of sensory systems and temperamental traits.