Plasma concentrations of glucagon-like peptide-1(7–36)amide (GLP) and gastric inhibitory polypeptide (GIP) were determined at fortnightly intervals for over a year throughout the pregnancy–lactation cycle of goats. Both GIP and GLP concentrations were elevated during lactation and fell rapidly when milk secretion was terminated. At the onset of lactation GLP concentrations rose rapidly whereas GIP concentrations showed a delayed response. GLP concentrations remained high throughout lactation but those of GIP declined linearly as milk yields fell. Serum insulin concentrations correlated positively with plasma glucose concentrations but not with either GIP or GLP concentrations. Negative correlations were found between serum insulin concentrations and milk yield and plasma non-esterified fatty acid concentrations. The results are consistent with plasma GIP and GLP concentrations being determined by other factors in addition to nutrient intake and absorption. Changes in GIP concentrations mirrored reported changes in the hypertrophy and atrophy of the intestine in ruminants while GLP concentrations may be more dependent on the neural and endocrine factors associated with lactation. The elevated concentrations of both peptides indicated a specific role in lactation independent of their normal anabolic and insulinotropic effects.