A group of pregnant control rats was sacrificed before parturition, in the morning, afternoon and evening of day 20 and 21 and in the morning of day 22. Another group was sacrificed during parturition, when 2 to 8 foetuses had been expelled. The onset of parturition occurred for the first rat in the afternoon of day 21 and for the last rat in the afternoon of day 22. Corticosterone was extracted from maternal, foetal and neonatal adrenals and plasma, and was assayed by a fluorometric procedure. The maternal adrenal and plasma corticosterone levels, before parturition, were lower in the morning than in the afternoon excepting day 22 when morning values were as high as those in the afternoon of day 21. Adrenal and plasma corticosterone concentrations were elevated during parturition in the mother but not the foetus. Plasma corticosterone values were raised in the newborn compared to their littermates in utero.