SummaryThese studies were implemented to assess the relationship between fetal vasopressin secretion and the progression of parturition as well as the contribution of specific stimuli to vasopressin release during labor. In chronically catheterized fetal lamb preparations, labor was induced by infusion of adrenocorticotropin (12.5 mg/kg/hr) to seven fetuses at 130 ± 1 day of gestation. Before labor, fetal plasma vasopressin concentrations were 2.1 ± 1.4 pg/ml and remained low (5.3 ± 3.4 pg/ml) during prodromal and early phases of labor, but rose significantly in the active and expulsive phases (39.6 ± 27.5 and 173.3 ± 152.9 pg/ml) to reach peak values at delivery (584.2 ± 433 pg/ml) and decrease by 30 min after birth (359.8 ± 90.0 pg/ml).At delivery, fetal plasma vasopressin concentrations were strongly correlated (P< 0.001) with hormone values obtained during the latter phases of labor. Fetal arterial pH and oxygen tension was inversely correlated with plasma vasopressin (P< 0.01). No similar correlations were found with arterial PCO2, K, Na, Cl, osmolality, or packed cell volume. Unexpectedly, we observed a significant (P< 0.001) and progressive decrease in fetal oxygen tension during the induction process. Other characteristics of adrenocorticotropin-induced parturition seemed to mimic those of spontaneous labor.