Studying the influences of particular environment in the mountainous farm village of Okunoto-district to serum electrolytes, especially to calcium, and the following results were obtained. 1. Serum calcium in the healthy farmers including both sexes demonstrated gradual alteration throughout the year with the minimum peak at May and June. 2. Serum electrolytes of the healthy male and female farmers were determined in both farming and leisure seasons for farmers. In the farming seasons, serum calcium was evidently decreased in both sexes compared with the leisure seasons for farmers, and, contrariwise, potassium was increased, and sodium was almost unchanged or slightly increased during the farming seasons. 3. It was supposed that such decrease in serum calcium during the farming seasons was not due to the influence of ultraviolet ray but to poor calcium intake by their eating habits and to fatigue as a result of the particular condition, the farming seasons. 4. In the control group of the healthy individuals other than farmers, serum calcium demonstrated almost no change throughout the year except for a very slight decrease during the periods of April, May and June. 5. In the school children, a decrease in serum calcium was noted during the winter time when ultraviolet rays were diminished. This indicated the fact that many babies suffering from rickets in this district. Such tendencies were also seen in the pregnant females. 6. A decrease in serum calcium due to diminution of ultraviolet rays were observed not only in the babies but in the school children and pregnant females except for the adults in good health. 7. A marked restoration of serum calcium level was noted 2 months after the administration of vitamin D and ultraviolet rays as a treatment of rickets.