Previously published studies point to the behavioral interaction between the mother and her litter as the origin of the processes responsible for the behavioral hyperactivity following maternal separation in 2-week-old rats. The following experiments explore (1) the time course over which behavioral hyperactivity develops, (2) environmental and social determinants, (3) the role of sensory stimulation in the regulation of behavior and (4) the olfactory sensory system as a mediator of separation effects. The hypotheses supported best by the data are (1) that tactile and olfactory stimulation repeatedly provided by the mother act to reduce levels of behavioral arousal of the infants and (2) that maternal separation exerts its effect by withdrawal of this regulating influence, allowing a new level of behavioral arousal, more intrinsic to the infant, to be revealed.