Summary. Plasma cortisol and liver glycogen were measured in the mother, fetus and newborn of guinea-pig after a stress on days 60 and 67 of gestation (parturition occurring on day 68) ; at those times, the animals were held for 3 h in the dark and subjected to a flash of 75 J intensity every 10 sec. In comparison with the controls, stress increased plasma cortisol in both mothers and fetuses. While it decreased maternal liver glycogen on day 60, it increased fetal liver glycogen at the same stage. Glycemia decreased in mothers after stress, although at delivery females previously stressed on day 60 showed hyperglycemia.Introduction.