We have analysed the sex difference of calcitonin (CT) levels after combined stimulation with calcium and pentagastrin (Ca-PG) in the normal population, and the relationship of the post-stimulatory CT levels with free testosterone (FT). We have also studied the correlation between CT values and the anthropometric parameters, body mass index (BMI) and body surface area (BS), as well as the relationship between CT levels and calcium. A positive and statistically significant correlation was found between post-stimulatory CT and the increment over the base-line of CT and basal FT, and with the anthropometric parameters. However, the increment of CT and the peak values of CT did not have any significant correlation with the Ca levels (basal or post-stimulation). We conclude that the enhanced CT response found in normal men compared to normal women is at least partially determined by the higher testosterone levels found among normal men.