Fetal rat pancreatic cells were isolated from pancreatic primordia on days 12-14 of pregnancy and cultured for 48 h in the presence of 5 mmol/l glucose. Insulin accumulation in the medium over the next 24 h was measured. Cultured cells from day 12 fetuses secreted about 1 fmol insulin per pancreas in response to 5 or 15 mmol/l glucose irrespective of whether 1 mmol/l tolbutamide, 400 mumol/l diazoxide, 5 mmol/l theophylline or 10 mmol/l mannoheptulose was present. In contrast, insulin released from day 13 cultured cells increased significantly from 3.0 +/- 0.6 to 6.2 +/- 2.2 fmol per pancreas, when the glucose concentration was raised. Tolbutamide increased, diazoxide and mannoheptulose decreased and theophylline had no effect on insulin release. Even more pronounced effects were found on insulin release from day 14 cultured cells, in which theophylline also increased the release. In addition, insulin release from cells from pregnancy day 14 was 75 +/- 16 amol/min per pancreas when the cells were perifused for 15-20 min in the presence of 5 mmol/l glucose within 3 h of isolation. Increasing the glucose concentration to 15 mmol/l or adding tolbutamide increased, whereas diazoxide decreased, insulin release in the freshly isolated cells. The insulin content of rat pancreata from pregnancy day 13 was 0.06 +/- 0.01 pmol per pancreas and increased approximately 10-fold every second day up to 6.7 +/- 0.9 pmol on day 17 of pregnancy. Between day 17 and 19 the pancreatic insulin content increased about fivefold to 39 +/- 2 pmol. The present data suggest that critical components of the insulin-secretory machinery, including ATP-regulated K+ channels, glucokinase and adenylate cyclase activities, are present in the developing beta-cell earlier than hitherto thought.