The relation between the reproductive cycles of Presbytis senex and P. s. monticola and environmental factors were studied in two climatically different areas of Ceylon: Polonnaruwa and Horton Plains, respectively. At Polonnaruwa, which is a seasonally dry area, the peak mating period coincided with the period of high rainfall, food abundance, decreasing temperatures and day length, giving rise to a birth peak during the dry season. The gestation period of P. s. senex was estimated, and intratroop breeding synchrony was noted within troops that underwent replacement of the leader male. At Horton Plains, where climatic variations were less marked, a distinct birth peak was not found. The attenuated peaks seemed to be due to intratroop breeding synchrony within established troops. The interbirth interval was 16-17 months, whereas at Polonnaruwa, it was extended to about 24 months.