The spontaneous activity of [male] rats was greatly increased and modified by close proximity [though in separate cages] to normal [female][female] in the prime of life, apparently owing largely to a sex-drive stimulus and little, if at all, to the increased activity of the adjacent [female][female]. The activity of a [male] close to a single [female] not only was increased but also approached closely the estrual rhythm of that [female]. Response to more than one [female] near by, whose estrual periods occur on different days may be to any of the stimuli, which results in a lack of rhythm and a further increased activity of the [male].