This study examined the effects of intrauterine growth on insulin secretion and resistance in newborn foals. Embryo transfer between small pony and large Thoroughbred mares was used to produce four groups of foals with different birth weights (pony in pony n=7; pony in Thoroughbred n=7; Thoroughbred in Thoroughbred n=8; Thoroughbred in pony n=8). On day 2 after birth, glucose (0.5 g/kg) was administered intravenously to the foal and blood samples were taken for 2 h to determine plasma glucose and insulin concentrations. On day 3, insulin sensitivity was assessed by giving insulin (0.75 U/kg i.v.) and measuring the decrement in plasma glucose in the foals. There were no significant differences in insulin secretion, insulin sensitivity or glucose tolerance between the control and growth-retarded Thoroughbred foals. Overgrown pony foals delivered by Thoroughbred mares had higher basal insulin levels and greater beta cell responses to glucose than the other groups of foals. The relationship between plasma glucose and insulin was also significantly steeper in overgrown pony foals than in the other groups. Variations in intrauterine growth rate, therefore, affect postnatal insulin secretion in the horse. More specifically, it is overgrowth, not growth retardation in utero that alters equine beta cell function in the immediate neonatal period.