The main endocrine function of the testis after puberty is testosterone production. In most cases, hypogonadism in adult men can be diagnosed by determining total testosterone concentration. Due to the circadian rhythm of testosterone secretion, blood samples should be extracted early in the morning. The results of commercially available methods for analysis show considerable variability. Furthermore, the threshold for the symptoms of hypogonadism may differ in each individual. For these reasons, moderately low testosterone levels should be interpreted with caution before a diagnosis of hypogonadism can be established. In these cases, determination of either free or bioavailable testosterone can be useful. Direct methods can be used or the respective concentrations can be calculated on the basis of total testosterone and sex hormone-binding globulin (SHBG). This latter method is easy to perform but the results are less reliable. Endocrinological evaluation of the testes should also include analysis of the gonadotropins (follitropin [FSH] and lutropin [LH]), which are described in another article in this series. Inhibin B is a biological marker of the amount and the physiological status of Sertoli cells in the postpubertal testis. Inhibin B may improve the information given by FSH for the determination of spermatogenic reserve in non-obstructive azoospermia, but determination of this glycoprotein is not currently used for routine assessment. The most important laboratory test to study reproductive function in men is semen analysis. However, the predictive power of this test is limited by the analytical imprecision of current methods, all of which are manual, and by the biological variability of most of their components. Special attention should be paid to pre-analytical procedures, because they require the understanding and participation of the patient. Some organizations and societies have proposed standardized methods to help improve the quality of semen analysis and reliable exchange of the results of seminogram. Biochemical markers of the prostate, seminal vesicles and epididymis in seminal plasma can indicate the level of damage in hypospermia or azoospermia. The fertility potential of sperm cells can be investigated with a variety of tests and assays, but none of them can yet be recommended for routine practice. Congenital hypogonadism is frequently caused by chromosome abnormalities, particularly sex chromosomal aneuploidies. Other causes of infertility include structural aberrations of autosomes. The main cytogenetic technique performed to determine chromosome constitution is karyotyping. To detect submicroscopic defects, this test can be performed in conjunction with fluorescent in situ hybridization (FISH).