The factors influencing β cell function in the foetal and neonatal foal have been investigated in chronically catheterised foetal foals and in newborn foals delivered either spontaneously at term or by induction at different gestational ages. Insulin was detected in the foetal plasma from as early as 150 days of gestation (term = 340 days) and during the last third of gestation the foetal /3 cells responded to exogenous administration of glucose and arginine and to endogenous variations in the glucose level. Insulin secretion by the foetal ft cells was depressed by anaesthesia and surgery. At birth, there was a significant positive correlation between the plasma concentrations of insulin and glucose irrespective of the maturity at birth or type of delivery (r = 0.86, n = 39, P < 0.01). The slope of this relationship was significantly less than that relating the postoperative foetal concentration but only when delivery was difficult or prolonged was the ft cell sensitivity to glucose completely abolished. At birth, there were no significant differences in the plasma concentrations of insulin or glucose between full term foals delivered spontaneously or by induction. However, the spontaneously delivered foals showed a transient increase in the insulin concentration 15 mins after birth which was not observed in the full term foals delivered by induction. Plasma glucose concentrations were maintained during the 2 h after birth in the absence of sucking in both the induced and the spontaneously delivered full term foals. Premature foals had significantly lower plasma glucose concentrations at birth than full term foals. Their plasma glucose levels also fell during the 2 h after birth. Exogenous infusion of glucose 2 h after birth elicited a rapid release of insulin in the full term foals but caused only a small and delayed ft cell response in the premature animals. These observations demonstrate that the pancreatic β cells of the foal are functional and responsive to a variety of stimuli during the perinatal period and that insulin secretion in the immediate post natal period is influenced by the maturity of the foal at birth.