The renin-angiotensin system is critically important to fetal cardiovascular function and organ development. The feedback regulation of renin secretion by ANG II develops early in gestation yet does not linearly progress from fetal life to adulthood. Renin secretion is elevated in late gestation compared with earlier or postnatal time periods, which suggests that some component of the negative feedback regulation of renin secretion is less sensitive in late gestation. We examined in fetal sheep the age-related consequence of chronic in vivo manipulation of ANG II on renal renin secretion measured in vitro. Immature (101-103 days of gestation) and mature (130-133 days of gestation) fetuses were treated for 72 h with enalaprilat, ANG II or vehicle. Content and basal and isoproterenol-stimulated secretion of prorenin (PR) and active renin (AR) from fetal kidney cortical slices were determined. Enalaprilat pretreatment in vivo increased renal renin content and basal and stimulated secretion of PR and AR in vitro even in immature animals. Immunohistochemical localization showed that enalaprilat treatment caused an age-related recruitment of renin-containing juxtaglomerular cells. Conversely, ANG II pretreatment decreased basal and stimulated PR and AR secretion from immature fetal kidneys, but only inhibited PR secretion from mature kidneys. It also caused an age-related decrease in the percentage of renin-containing juxtaglomerular cells. These results suggest that ANG II feedback modulates not only the synthesis and content of renin, but the sensitivity of the coupling between stimulus and secretion. A critical observation of our study is that the higher renal tissue concentrations of prorenin and active renin in late gestation may be a consequence of reduced sensitivity to ANG II feedback; this is consistent with the increased plasma concentrations of renin found in near-term mammals.