I J The chronic administration of isoproterenol(5 mg./kg. twice a day) to male and female mice resulted in increased wet weights and tissue weight-body weight ratios of the submaxillary and parotid glands. For the heart the ratio was elevated in male but not in female mice. The first dose of isoproterenol produced a decrease in the rectal temperature. Continuation of the treatment led to hyperthermia, which became maximum after treatment for longer than 10 days (20 doses). In these animals the predose rectal temperatures were lower than the pretreatment control values and the temperatures of control animals. A smaller, single dose (2.5 mg./kg.) did not alter the rectal temperature. The norepinephrine concentrations of the parotid and submaxillary glands were reduced in male and female mice, but the total norepinephrine content of these glands was decreased only in male mice. The cardiac norepinephrine levels were not affected. Chronically treated animals were less active than controls for about 90 min. after dosing and showed rarification of fur and hair loss. Isoproterenol treatment of dams before, during, and after pregnancy did not alter the body weight, gross appearance, and wet weight of organs and tissues of pups examined on the 3rd postnatal day.