IN previous investigations we have attempte,d to demonstrate the relationship of climatic variations during the first trimester of pregnancy to the b,irth of mentally deficient children. In the most recent study we tested the hypothesis that above-average summer temperatures during the third month of pregnancy, the critical period of fetal central nervous system development, are associated with an increased risk of being born mentally defective. This hypothesis was substantiated for the period of 1913-48 based on the date of birth, and therefore of conception, of mentally defective children admitted to the Columbus State