The postnatal development of Peyer's patches was investigated using routine histological and immunohistochemical techniques, focusing especially on the formation and increase in number of lymphoid follicles. In addition, we studied the influence on the development of lymphoid follicles in Peyer's patches of a sublethal dose of whole-body X-irradiation at an earlier postnatal age. At 1 day after birth, surface-IgM (sIgM)-bearing cells (B lymphocytes) were scattered throughout the Peyer's patch. At Day 3, sIgM-bearing cells had accumulated to form primary follicles in association with domed elevations. OX2-positive reticular cells (FDC) were detected in the centers of primary follicles at Day 5. The first appearance of germinal centers within lymphoid follicles was noted at 18 days, and at 21 days almost every follicle contained a germinal center. At Day 5, each Peyer's patch contained 6-8 lymphoid follicles, with the lymphoid follicles subsequently increasing in size and number. The mean number of follicles per Peyer's patch became 11.1, the adult level, at 21 days, and remained at this level for the next 15 weeks, even though respective follicles continued to enlarge during the observation period. Three-week-old rats received 400 rad whole-body X-irradiation. At 3 and 7 days after treatment, lymphocytes were largely depleted from Peyer's patches, leaving behind the structural framework of the lymphoid follicles, interfollicular zones and domes. Stromal cells in the follicle remnants retained a positive reaction to OX2.(ABSTRACT TRUNCATED AT 250 WORDS)