The comparative growth patterns of endocrine gastrin and somatostatin cell populations were examined in the rat, during the perinatal period, to investigate possible relationships between their development and that of gastric acid secretion, gastrin and somatostatin hormones being implicated in the regulation of acid secretion. Total cell populations were estimated daily in the pancreas, stomach, and duodenum, by using a quantitative morphological method, from 19 days postcoitum to 8 days postpartum. In the pancreas, both cell types were present at 19 days postcoitum. After increasing, gastrin cells abruptly dropped from 4 days postpartum, while somatostatin cells continued to increase. In the stomach, gastrin cells seemed to appear at 19 days postcoitum, increasing with age. Somatostatin cells appeared only after birth and could be precisely quantified from 4 days postpartum. In the duodenum, the two cell types were present in similar numbers at 19 days postcoitum and increased similarly with age. Comparison of gastrin and somatostatin cell developmental behavior with previous data on the ontogeny of acid secretion shows a parallelism between the appearance of basal H+ fluxes at 20-21 days postcoitum and the high daily multiplication of the gastrin cell number in the three organs. Additionally, the marked decrease of pancreatic gastrin cell population at 4 days postpartum and the simultaneous development of the gastric somatostatin cell population might explain, among other mechanisms, the diminution of gastric acid secretion noted after birth.