Six collections of adult Collocalio maxima Hume, including both sexes, were taken at the nest sites in Niah cave, Sarawak (113° 46' E. 60' N). Changes in the gonads and accessory reproductive organs of these specimens indicate a prolonged breeding season of eight months, September to April annually, after which the gonads regress to nonbreeding condition. The moult is also a seasonal event, but the progress of moulting in individual birds is not directly related to breeding condition. The sublingual glands of a fledgling and of certain adults were minute and nonsecretory. It is concluded that the secretion of these glands has no digestive function, but is related solely to periodic nest building. No correlation is found between the reproductive state of individual birds and activity in the sublingual glands. Some females in breeding condition had totally inactive sublingual glands, whereas all males in breeding condition had active sublingual glands. It is suggested that these glands may be responsive to teaticular hormone. The most important external environmental factor controlling the activity of the sublingual glands appears to be the presence or absence of the nest itself.