Infertility is associated with oxidative stress, normally counterbalanced by different antioxidant systems. In order to explore the hormonal control of seminal plasma total antioxidant capacity (TAC) we evaluated TAC and hormone patterns in a group of unselected infertile patients and control subjects. One hundred and ten infertile patients (divided into 3 groups: inflammation, varicocele, and other etiologies) and 31 fertile men were examined, evaluating blood serum gonadotropins, testosterone, estradiol, free tri-iodothyronine, free tetraiodothyronine (FT4), thyrotropin, prolactin (PRL), seminal parameters, and TAC. TAC was measured using the H(2)O(2)-metmyoglobin system, which generates the spectroscopically detectable radical cation of the chromogenous compound 2,2(I)-azinobis (3-ethylbenzothiazoline-6-sulfonate). The "lag time" of its appearance is proportional to the antioxidant activity. Lag phase was significantly higher in varicocele vs controls, whereas it was lower in patients with inflammation vs varicocele or other kinds of infertility. The correlation analysis between hormones and seminal parameters showed an inverse correlation between PRL and sperm motility, and a direct correlation of TAC with PRL and FT4, but not with gonadotropins or gonadal steroids. Our data suggest that systemic hormones may play a role in regulating seminal antioxidant capacity. This is interesting also because some hormones, such as thyroid and pituitary hormones, are not usually tested in the first-level evaluation of male patients with fertility problems.