Flying-foxes (genus Pteropus, suborder Megachiroptera) are long-lived tropical mammals. Their seasonal reproduction appears to be regulated by an endogenous, circannual rhythm modified by multiple environmental cues. Luteinizing hormone (LH) content in pituitary extracts was examined to establish the broad time-frame of pituitary stages in the reproductive seasonality of the flying-foxes. A comparison was made between the grey-headed flying-fox P. poliocephalus, which mates and conceives in autumn, and the little red flying-fox P. scapulatus, which mates and conceives in spring. In P. scapulatus, LH was maximum during the spring mating season at 1494 ng mg^-1 in males and 896 ng mg^-1 in females. In P. poliocephalus males, LH increased to 1082 ng mg^-1 in early summer, 4 months before the mating season; LH concentrations in male P. poliocephalus returned to a low of 222 ng mg^-1 by the time of the autumn mating, by which time the female P. poliocephalus expressed elevated LH concentrations (624 ng mg^-1). Apparently in P. poliocephalus, the peak LH concentrations in females are delayed by 4 months relative to LH concentrations in males. This is associated with 4 months of energetic courtship on the part of male P. poliocephalus, which is not observed in P. scapulatus, the fertility of which is synchronized between the sexes. The heterologous radioimmunoassay developed using monoclonal antibody 518B7 confirmed classic suppression of LH during pregnancy and lactation in flying-foxes and LH elevation in response to gonadectomy. Juveniles generally had pituitary levels similar to non-breeding levels in adults.