Summary. The major reproductive events in the oestrous cycles of nonhibernating mega- and microchiropteran species are reviewed. However, special attention is given to the reproductive biology of the temperate North American species, Tadarida brasiliensis mexicana (Molossidae), the Mexican free-tailed bat, which expresses dextral uterine and ovarian dominance. Only the larger right ovary is capable of producing an ovulatory follicle and the left has long been considered atrophic. In order to elucidate the normal oestrous cycle and define the structural and functional characteristics of the ovaries of this nonhibernating bat several analytical techniques were utilized. These included light microscopic analysis, histochemical localization of Δ5-3β-hydroxysteroid dehydrogenase (HSD) activity, and radioimmunoassay of seasonal plasma progesterone levels. Interstitial tissue was found in both ovaries, but the left was almost entirely an interstitial organ. Histochemical analysis demonstrated that the gonads have seasonally varying amounts of Δ5-3 β-HSD localized either in the thecal cells of the Graafian follicle or in the interstitium. The corpus luteum persisted throughout gestation, reaching maximum development just prior to parturition. Circulating progesterone levels correlated directly with luteal gland size and peaked at 106 ng/ml when the corpus luteum was largest.