Locomotor activity was recorded from three squirrel monkeys, Saimiri sciureus, housed singly in cages inside a sound-proof chamber. Each animal was exposed twice to each of three conditions; continuous dim illumination (dim LL), continuous bright illumination (bright LL), and conditions in which the animal could turn on bright light by itself (self-controlled light-dark cycle: LDs). The mean circadian period, tau, the activity time, alpha, and the amount of activity, A, were computed for each single condition. It was found that, in LL, tau, alpha and A were positively correlated with the intensity of illumination. In LDs, tau was longer and A larger than in either dim or bright LL. The lengthening of tau in squirrel monkeys by a self-controlled light-dark cycle is compared with similar findings in birds and man, and is discussed in view of the observation that the tau-characteristics of diurnal mammals deviate from those known from other diurnal species of vertebrates.