Results of a previous investigation of manic-depressive psychosis with a continuity of cycles of short duration (Klein and Nunn, 1945) seemed to justify further study of this group. As such cases suitable for investigation are rare, we are again reporting observations on a single case, although the cycles were not as short and regular as in the earlier case. The patient at the time of his last admission to this hospital (in 1944) was aged 40, married, had a healthy child of 16, and was an employee of a tobacco factory. He was said to have been always very quiet and reserved, and had no history of mental illness before joining the Army in 1941. He went to the Middle East in July, 1942, and began to feel listless, tired and sleepless for the first time at the beginning of 1943. After spending four weeks in a military hospital he still felt low on returning to his unit, but soon afterwards was full of life; in his own words, “I could tear anything down.” A fresh period of depression set in later, and he was sent to a military mental hospital for six months, during which time he apparently changed several times from a depressive state into a manic one in which he felt “overstowed with vitality,” “with the energy of ten men.” From this hospital he was returned to England and invalided from the army in October, 1943, at which date he first became a patient in this hospital. An initial depression of about three weeks was followed by a manic condition, and on becoming more settled he took his discharge (at the beginning of December, 1943). In February, 1944, he was readmitted in depression, but went out again whilst in moderate mania in April. Whilst at home he continued his work in the factory, but in September, 1944, was once more admitted to hospital and has since remained. From the latter date till October, 1948, depressive periods have alternated with manic conditions. Wide variations have been noted in the duration of the two phases; at the beginning of the observation period of 21 cycles the manic state lasted longer than the depression, whilst in the last year the reverse was usually true. Manic periods varied between one week and 35 days, with an average of 18 days, while the variation shown by depressive phases was between 6 and 24 days, with an average of 13 days.