The effects of methylphenidate were compared with those of a placebo in 70 senile patients committed to a state institution because of mental disturbances. Neither the patients nor those who rated their behavior (orientation, appetite, continence, cooperativeness, activity, rationality, personal care, and irritability) knew whether a given patient was receiving the drug or the placebo. Among those who received the drug, two dosage schedules were used, the doses being increased weekly until some patients were receiving 60 mg. daily during the sixth week of the study. No significant differences were found between the test and contratest groups as regarded weight, pulse rate, or behavior ratings. A statistically significant lowering of the blood pressure was found among the subjects who received a maximum daily dose of 30 mg. of methylphenidate but was not found among those who received the maximum daily dose of 60 mg. No undesired side-effects were observed in this study, but the drug in the dosages used did not have any convincing effect on the physical or mental status of the patients.