Hypoacidity and hypergastrinaemia are reported in the newborn human. To investigate the ontogeny of gastrin, plasma gastrin concentrations were measured in chronically cannulated foetal sheep from 100 days gestation to term (145 days) and up to 16 days following delivery. Plasma pancreatic polypeptide (PP) concentrations were measured as a marker of vagal activity. Compared with adult values, foetal plasma gastrin concentration was low at 101-105 days, being 7 +/- 1 pmol/l (mean +/= SEM) but greater than adult values from 130 days gestation and increased to 47 +/- 5 at 141-145 days, and 90 +/- 13 at 1-5 days post-partum. Maternal plasma gastrin level during these periods ranged between 21 and 29 pmol/l and was not related to gestational age. Similarly, maternal plasma PP levels, which varied between 220 and 400 pmol/l, did not correlate with gestational age and did not differ significantly from non-pregnant sheep. Foetal plasma PP was low, 20 +/- at 101-105 days, rose to 92 +/- 17 at 141-145 days, and increased to adult levels in the first week post-partum. Basal foetal and maternal plasma PP were inhibited by iv atropine injection. The increase in plasma PP may represent a maturity of vagal influence. Gastrin and PP have a trophic action on the gastro-intestinal tract, so the observation of significant levels of circulating gastrin and PP in the foetus suggests that they may be involved in maturation of the gastro-intestinal tract.