The seasonal variations of eighteen parameters of a biochemical profile were studied under standardized conditions. More than 2,600 blood samples from a homogeneous group of women aged 30 to 39 years were processed on a well controlled instrument over a four-year period. Monthly means were first calculated and then seasonal means for two delimitations of seasons and for different combinations of years. Even though many statistically significant differences were found between seasons, only in the case of two parameters, uric acid and triglycerides, was the percentage of difference between the highest and the lowest seasonal means greater than the inter-assay CV of the methods. For these two parameters the results of other age groups of women and men were also computed. Uric acid was higher in summer compared to fall by 5% in the case of women aged 30-39 and by 7% in the case of men aged 30-39, the method inter-assay CV being 4.5%. Triglycerides were 6% higher during spring compared to fall for women aged 30-39, but surprisingly, 22% higher in winter compared to fall in the case of men aged 30-39. For younger groups of men the difference is much smaller and for men aged 20 to 25 years the difference is not significant. In general, we have been unable to confirm the variations previously reported by many authors.