The effects of constant darkness, pinealectomy, and heterotopic pineal transplantation on the increase in body weight and on sexual maturation were studied in prepuberal female rats. Animals on restricted food intake served as additional controls. caused a subnormal increase in body weight, subnormal ovarian weight at puberty, and in some but not all Light conditions experiments delayed vaginal opening. All these effects seemed to be dependent on the presence of an in situ pineal gland. Observations on animals on a restricted food intake and data on animals varying spontaneously in body weight increase indicated that a smaller body weight increase causes a delay of vaginal opening but not a subnormal ovarian weight at puberty. It is argued that, in maturing animals, the anti-gonadotrophic and anti-growth influences of the pineal gland are separate physiological mechanisms.