T4 and T3 are iodoamino acid hormones produced by the thyroid gland. The biological half life (t_1/2) of T4 is 4-5 times that of T3 as well as T3 binds to the receptor of target cells with 10 times as the affinity of T4 (Schimmel and Utiger, 1977 and Martin et al., 1985). Moreover, the majority of circulating T3 arises by the peripheral monodeiodnation of T4 in the liver and kidneys (Sterling et al., 1977). In farm and laboratory animals, hypothyroidism or thyroidectomy resulted in degeneration of the testis, arrest of spermatogenesis as well as depress- ion of spontaneous activity, motility, viability and increased abnormality of spermatozoa (Swanson and Boatman, 1953, Bruni et al., 1975 and Ghorieb et al., 1978). On the other hand, thyroid therapy was suggested to improve reproductive performance of the male animals (El-Azab et al., 1974). The metabolic energy required for survival and motility of spermatozoa is mostly gained from seminal fructose through fructolysis. Moreover, an intimate relationship was found between fructolytic rate and spermatozoal density and motility in bull semen (Mann and Lutwak-Mann, 1981). Available literature concerning existence and content of T4 and T3 in semen of bulls are scarce. Therefore, this study was planned to determine levels of serum and seminal T4 and T3 as well as their relation with semen quality.