Four normal persons, two men and two women, have served as subjects for this investiagtion. On each, the following observations were made once a week throughout the year (February, 1925—February, 1926) under basal conditions between 8 and 10 a. m. The observations and their apparent course throughout the year follow:1. Oral temperature; no certain periodicity.2. Reclining systolic pressure: all show a marked depression in the spring and in three cases there are also low points in midsummer and late fall—early winter.3. Reclining pulse: lowest in summer.4. Alveolar air:(1) Oxygen, uncertain, with perhaps a tendency to be lowest in summer;(2) Carbon dioxid, highest in the summer in all subjects and rhythmically coincident with the menstrual cycle in the women, the low points coming just before the onset of menstruation.5. Pulmonary ventilation: both the tidal and minute volumes show a marked depression in the spring with a rise to maximum in late summer and early fall.6. Expired air: the percentages of oxyg...