SUMMARY Tissue and plasma insulin concentrations were measured in foetal and postnatal rabbits. Insulin was detected in 18-day foetuses and pancreatic insulin concentrations from the 22nd day of foetal life onwards were similar to one another. Insulin was detected in foetal plasma from the earliest time that foetal blood could be collected: 20 days. Foetal plasma insulin levels did not change significantly with the length of gestation but maternal levels were lower on day 30 than on day 20. There was a positive maternofoetal plasma insulin gradient on day 20 and a negative gradient on day 30.