One hundred and three patients treated for endogenous depression with unilateral, non-dominant ECT, half with two, half with four treatments weekly, were examined to determine the effect of the treatment on memory functions. Th first-mentioned group was studied with double-blind technique in connection with an earlier study while the latter group was investigated under open conditions. Tests were made before the first treatment, after the sixth treatment, and after the final treatment (the Wechsler Memory Scale Forms I and II) and at the same times, ratings were made of the depressive state. No significant differences were found between the results of the two groups either with regard to the total results or the sub-test results in WMS. This was true for changes between and the first and the sixth treatment and between the first and the final treatment. On the whole, both groups showed improved results during the course of treatment; a memory reduction to an equal degree in both groups evident before treatment, and thus due to the depression, was eliminated concurrently with the improvement in the depressive condition. With reference to delayed reproduction a comparison made after the sixth treatment showed that the high-frequency treatment group scored significantly higher than the other group. This can only be ascribed to a better therapeutic effect resulting from accelerated treatment. Likewise these improved therapeutic results in the high-frequency treatment group must be responsible for the fact that this group does not show poorer test results than the group treated at the customary frequency-rate in spite of an expected deteriorating effect of accelerated treatment on memory functions and in spite of a larger number of treatments per series in this group. Neither in the higher nor in the lower frequency treatment group were any age-related variations in memory found.