Serum lipids and thyroxine were determined in 26 men participating in a 90-km cross-country ski race, before, immediately after, and on the following days. Serum cholesterol was unchanged immediately after the race, but then fell significantly and remained low during the observation period. During the race the fatty acid composition of serum free fatty acids changed towards the composition of ordinary adipose tissue. The compostiion of serum triglycerides showed similar but less pronouced changes. On the following days an increase in the arachidonic, and a decrease in the linoleic acid fraction of the total serum lipids were observed. Total thyroxine and free thyroxine in serum were significantly increased at the end of the race, but had returned to the pre-raced levels during the rest of the observation period. The observations correspond to the findings during acute, severe illness, and are compatible with the hypothesis that some of the changes in serum lipids following severe somatic stress are due to increased thyroid hormone activity.