The present study has attempted to determine the effect of electric shock treatment on mental efficiency. The population studied included 75 patients, suffering from depression, hypomania, or psychoneurosis. The mean age was 41.2 years ; the mean number of shocks was 7.8. On Babcock tests administered an average of 11 days after shock, there was a slight but not significant improvement. On the Shipley test, the improvement was probably significant. When the same tests were administered after a follow-up interval averaging 189 days, there was relatively great and statistically significant improvement on both tests. Babcock scores obtained by the patient group about 6 months after shock were not significantly lower than the scores of a normal population of 17 persons of the same age and vocabulary level. It may be concluded that, for the population studied and under the conditions of the present experiment, electric shock treatment does not produce any significant impairment of mental efficiency after an interval of 6 months. No correlations were found between age and improvement from test period to test period. There was some evidence that patients receiving a greater number of shocks were affected more in mental efficiency than those receiving a lesser number of shocks.