On the basis of the results in vitro, it seems that the testicle does not respond to the action of the thyroid hormones. The study of the relationship between the blood concentrations of these hormones and the reproductive function of the organ is being made by the Authors as a means of verifying this in vivo. The total blood concentrations of T4, (triioothyronine), and the index of free Thyroxine (IT4L) were therefore evaluated both in normal and infertile subjects. The concentrations of T4 showed a correlation both with the sperm count (r = 0.43, p = less than 0.02), and with the percentage of motile spermatozoa, (r = 0.48, p = less than 0.01). This type of correlation is not found, however, either for the T3 or for the IT4L. These data indicate that the function of the thyroid gland is linked to that of the hypothalamo-hypophyseotesticular axis. It would appear that the peripheric metabolism of the circulating hormones is not involved.