The pattern of changes in the population of healthy and regressing follicles in relation to the estrous cycle, was studied in the recently domesticated desert gerbil, Meriones hurrianae, which exhibits a longer cycle (proestrus: 1.5 days; estrus: 1 day; metestrus: 1.5 days; diestrus: 3 days). As the granulosa cell layers were not distinct, the follicles were classified on the basis of their morphology and size (group I: small preantral follicles; group II: large preantral follicles; group III: small antral follicles; group IV: medium-sized antral follicles; group V: large-sized follicles; group VI: Graafian follicles). Regressing follicles were classified likewise. Five animals were used for each stage of the cycle. One ovary from each gerbil was serially cut and stained with hematoxylin-eosin. Healthy and regressing follicles were found throughout the cycle. The follicular development occurred typically in a biphasic manner. In phase A, in which follicles reach the antral stage, the growth of oocytes and follicles is linearly correlated, whereas, in phase B, oocyte size remains stationary while follicle size increases due to accumulation of antral fluid. However, oocyte growth and nucleus growth are positively and linearly correlated (r = 0.74). The healthy follicles under 260 microns (groups I, II) do not fluctuate in the ovary of cyclic gerbils, whereas follicles in group III (261-350 microns) increase significantly (p < 0.05) at estrus and those of group IV in diestrus. Further, healthy group III follicles are reduced in diestrus while regressing follicles in group III increase. These findings thus suggest that the antral follicles are rescued from atresia at estrus.(ABSTRACT TRUNCATED AT 250 WORDS)