Most studies on the relationship between the pineal gland and reproductive function have been concerned with the effects of pinealectomy or pineal extracts on reproduction (for review see Kitay & Altschule, 1954), but the effects of the state of the reproductive system on the pineal gland have been little investigated. The effects of castration have been studied by Borell & Örström (1947) and Quay & Levine (1957), and the effects of sex hormone administration by Brewer & Quay (1958). However, no studies seem to have been made of the pineal body during pregnancy in the rat. Fifty female rats, aged 3 months, were mated with males of similar age and thirty-eight became pregnant. The 1st day of pregnancy was defined as the 1st day on which sperms were found in vaginal smears, or vaginal plugs were seen. Twenty-five female rats of the same age and similar body weight served as