The effect of a lacto-ovo vegetarian (V) and a mixed, meat-rich (M) diet on the level of serum sex hormones, gonadotropins, and endurance performance of eight male endurance athletes was investigated in a 2 ± 6 wk cross-over study. The energy contribution from carbohydrate, fat, and protein was 58%, 27%, and 15% on the V diet and 58%, 28%, and 14 E% on the M diet. For total fasting serum testosterone (T) there was a significant interaction between diet and time (P < 0.01). Thus, the V diet resulted in a lower total T level (13.7, 9.8–32.4 nmol·l−1) (median and range) compared with the M diet (17.4, 11.8–33.5 nmol·l−1). During exercise after 6 wk on the diets total T was also significantly lower on the V than on the M diet (P < 0.05). Serum free testosterone, however, did not differ significantly during the 6 wk dietary intervention periods and neither did serum concentrations of sex hormone binding globulin, dihydrotestosterone, dehydroepiandrosterone sulphate, 4-androstenedione, estrone, estradiol, estrone sulphate, or gonadotropins. Endurance performance time was higher for six and lower for two after the mixed diet compared with the vegetarian diet. This was not significant, however. In conclusion, 6 wk on a lacto-ovo vegetarian diet caused a minor decrease in total testosterone and no significant changes in physical performance in male endurance athletes compared with 6 wk on a mixed, meatrich diet.