The effect of the neuro-spinal cord injury upon testicular physiology was evaluated in six adult paraplegic (PPG) men by measuring the circulating levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), androstenedione, testosterone, and dihydrotestosterone every 4 hr throughout a 24-hr period. Three PPG men were studied within the first 3 months (acute period) and the other three patients 39-79 months (stabilized period) after trauma. Hormonal values were compared with eight age-matched normal adult males. Plasma FSH and LH were constantly above normal concentrations regardless of the sampling time and period of observation, whereas prolactin was higher than normal only during the first two months after trauma, returning to normal afterwards. Plasma androgens were consistently below normal during the first 3 months after injury, and returned toward normal thereafter. There may be a direct relationship between the time elapsed after the spinal cord injury and the plasma androgens concentrations. A possible role of PRL in testicular steroidogenesis is suggested.