Day 9 mouse embryos were cultured in 50% fetal calf serum and 50% Waymouth's medium supplemented with various amounts of insulin. Unmodified medium contained approximately 25% of the normal adult mouse blood insulin. The addition of 0.1 to 1.0 Μg insulin/ml to Waymouth's medium was most beneficial to in vitro development. There was a statistically significant increase in protein synthesis, RNA content, glucose utilization, and heart rate, as well as a significant decrease in pericardial edema. DNA, synthesis was unchanged or slightly decreased. The optimal amount of added insulin (0.05 Μg/ml) is much higher than the normal circulating levels in adult mice.