The changes in Leydig cells of testes were studied in rats inflicted with 30% third degree scald. Light and electron microscopy, enzyme histochemistry of 3 beta-hydroxy steroid dehydrogenase (3 beta-HSD) determination of the relative activity of 3 beta-HSD, and assay of serum testosterone and luteinizing hormone (LH) were dynamically made for 30 days. It was found that there was various degree of degeneration and necrosis of Leydig cells. The activity of 3 beta-HSD in Leydig cells was rapidly reduced and remained at a relatively low level on the 30th day after scald. The serum level of testosterone was decreased rapidly and remained at a comparatively low level on the 30th day after scald. No significant changes in serum luteinizing hormone were observed. The results suggest that decreased in serum testosterone after scald might be the result of damage of the Leydig cells and increase in glucocorticoids.