L-Thyroxine (L-T<sub>4</sub>) was injected into mature rats daily at levels of 0, 3, 6, 12, 24, 48 and 96 µg/100 g body weight through estrous cycling, conception, pregnancy, parturition and 20 days of lactation. Mothers which lactated to 20 days were killed and mammary glands measured for DNA and RNA content. Those which were not able to maintain their pups were killed on the day when all pups died. Estrous cycling and pregnancy were not markedly effected by exogenous L-T<sub>4</sub> at levels 3–96 times the normal thyroxine secretion rate in rats. After parturition the mothers on L-T<sub>4</sub> above 12 µg did not allow the pups to suckle. As a result the pups died within 2–7 days after birth. Levels of L-T<sub>4</sub> from 3 to 12 µg allowed lactation to progress, but survival of pups to day 20 of lactation was significantly lower than in the normal control group. The results of this study indicate that high levels of L-T<sub>4</sub> inhibit mammary growth and milk secretion. This, in turn, results in the loss of pups, probably through depletion of prolactin as a result of higher metabolic rate due to hyperthyroidism.