During the course of other work by two of us, on a considerable group of lemurs and lorisoids in captivity in New Haven, Connecticut, certain types of cyclical behavior have become apparent. Since it has been necessary to reconstruct the living quarters of some of these animals, the disturbance of which precludes further observations in the immediate future, the more remarkable aspects of this cyclical behavior are reported at the present time, largely in the hope of stimulating other workers who may have access to prosimians to watch for comparable phenomena. The animals observed consisted of a group of four (2 c e, 2 9 9 ) Lemur fulvus E. Geoffroy, a pair of Lemur albifrons E. Geoffroy, 3 9 9 and 1 S Galago senigalensis E. Geoffroy, one c? and one immature 9 G. crassicaudatus E. Geoffroy, and three pairs Perodicticus potto (P. L. S. Muller). These animals lived from three to twelve months, up to December 1, 1961, in a room at the top (fifth floor) of a low tower with windows on all walls. The most important animals, 2 ce ce of Lemur fulvus rufus Audebert, one 9 of L. f. rufus ("Calo"), and one of L. f. fulvus ("Sal") occupied a large section of the room, could reach windows looking north and east, could easily see out from windows looking south and less easily from one small window facing west. A pair of pottos and both G. crassicaudatus occupied a similar section; the L. albifrons and the G. senegalensis lived in smaller but naturally illuminated cages, the other two pairs of pottos in darker cages. These animals are referred to collectively as group 1. A pair of Lemur fulvus rufus kept in the kitchen of a dwelling house with a window facing west-north-west and with little view of the night sky has served as a control. These animals are designated as group 2. All these animals are reported to breed seasonally. The lorisoids breed biannually, producing young about April and October. lGestation lasts between 3 and 4 months for G. senegalensis2-4 to 6 months for Loris.' Lemurs have a single annual breeding period. Mating occurs in the southern hemisphere during April, May, and June and parturition from September through January.5 In the northern hemisphere the seasons become reversed with birth occurring about April and May.", 5 Pregnancy lasts about 4 months. , 5 Oestrous cycles of 4 to 6 weeks are well documented in G. senegalensis,4 6 in Loris,l and in the lemuriforms Microcebus and Cheirogaleus.5 8 However, there is no record of a 4to 6-week oestral cycle in Lemur, although periods of swelling and