We have investigated glucose homeostasis and insulin response to glucose in seven rats before, during and after pregnancy, who were previously successfully transplanted with a single fetal pancreas. Increased need for insulin during pregnancy provides an opportunity to test the reserve capacity of the transplanted organ. Plasma glucose in seven rats was normal before pregnancy (7.3 +/- 0.7 mmol/l), during pregnancy (6.6 +/- 1 mmol/l) and after parturition (6.7 +/- 0.3 mmol/l). Fasting plasma glucose was lower after parturition (5.1 +/- 1 mmol/l) than before pregnancy (6.1 +/- 0.7 mmol/l). The disappearance rate of injected glucose was the same before (2.3 +/- 0.2%/min) as after pregnancy (2.6 +/- 0.2%/min). Basal plasma insulin before pregnancy was elevated and there was no rise from glucose; after parturition the basal and pattern of response was normal. The total insulin content of the transplants (859 +/- 154 mU) was only 21% of that of normal rats; we conclude that this provides a reserve adequate for the needs of pregnancy.