Effects of oral intake of nitrates on selected biochemical and endocrinological indices and its impact on reproductive functions were investigated in five feeder bulls aged 16-18 months. The bulls were tested prior to (30 days), during (30 days) and after (35 days) the period of the nitrate administration. The initial dose of 100 g potassium nitrate per day was increased at weekly intervals by 50 g up to 250 g per day. The administration of nitrates resulted in a highly significant (P < 0.01) increase in methaemoglobin concentration and a non-significant decrease in the concentration of beta-carotene and a highly significant (P < 0.01) decrease in the concentration of E vitamin in blood serum. A significant (P < 0.01) increase in blood serum concentration of bile acids and prolonged biological half-life of progesterone were suggestive of an impairment of liver metabolism. Prolonged intake of excessive doses of nitrates resulted in a significant (P < 0.05) increase in cortisol concentration during and after the administration period, while depressed thyroid gland activity was evident from a significant (P < 0.05) decrease in thyroxin concentration during the administration period. A suppression of hypothalamic functions after the administration period was documented by non-detectable levels (< 0.001 microgram/ml) of thyrotropin in TRH test. Depressive effects of nitrates on the function of Leydig cells during and particularly after the administration period were apparent from weakening testicular responses to a treatment with GnRH. Biochemical analyses of seminal plasma revealed a highly significant (P < 0.01) increase in total acid phosphatase activity and a significant (P < 0.05) decrease in the concentration of fructose. No other significant changes in seminal plasma components were observed. Adverse effects of excessive intake of nitrates were also evident from reduced sperm motility in the 120-min thermal test. While no difference was found in the frequency of primary morphological abnormalities, the number of secondary abnormalities rose by 115% in the post-administration period and was suggestive of damaged membrane integrity. Histological examinations revealed degenerative lesions in cells of the spermiocyte and spermatid layers.