The dynamics of insulin secretion during development of the fetal rat pancreas were investigated. The time of onset of glucose-induced insulin secretion was of special interest. Pancreases from 15- to 22-day-old fetal rats were perifused in vitro with low (0.5 or 0.9 mg/ml) or high (5 mg/ml) concentrations of glucose in the presence or absence of arginine and leucine. Levels of insulin in the perifusate were determined by radioimmunoassay. At day 17, a significant increase in perfusate insulin level was observed in response to arginine and leucine (each at 5 mM), This response was independent of a high concentration of glucose. In addition, perifusate insulin levels were augmented when the concentration of amino acids were kept constant and the glucose concentration was changed from a high level to a low level. On day 20, a monophasic, rapid-onset short-duration rise in insulin release with a high glucose concentration was observed. This response was enhanced by acetylcholine (2.7 x 10(-9) M). At days 21 and 22, insulin levels rose rapidly in the presence of high glucose and remained elevated. The results show that there is considerable precision in the timing of the onset and maturation of the glucose-induced insulin secretory response prenatally and reaffirm that insulin secretion by the fetal beta-cell varies with the stimulus applied.