A battery of clinical laboratory evaluations was obtained biannually in hyperactive boys treated continuously with methylphenidate hydrochloride for 1 to 4 years. Blood pressure and heart rate were evaluated annually. Pretreatment baseline studies did not implicate low blood glucose or calcium levels, or anemia, as factors contributing to hyperactivity. Prolonged administration of methylphenidate hydrochloride was not associated with disturbances in the hemopoietic, endocrine, hepatic or cardiovascular functions analyzed.