Different environmental factors affect animal physiological status. Day length and temperature are the two main factors that have been proved to cause changes in different hormones. The current study aimed to compare seasonal changes of some hormones such as testosterone (T), estradiol-17-β (E 2) and thyroid hormones (T 4 , thyroxine; and T 3, triiodothyronine) in selected vertebrates (amphibians, fish and mammals). These hormones were assayed by enzyme-linked immunosorbent assay (ELISA). The present results indicated that; T 3 had the highest concentration in both sexes of amphibians during winter and the lowest one was found in the investigated mammals of both sexes during summer and autumn. Regarding, T 4 also showed the highest concentration in both sexes of amphibians during winter and the lowest one was recorded in male mammals during summer. In relation to E 2 , in females; the highest and detectable concentration was observed in fishes during summer and the lowest one was found in mammals during winter. On the other hand, in males, its highest concentration was found in birds during spring, and the lowest one was recorded in amphibians during winter. Testosterone showed the highest concentration in female birds during summer, while the lowest one was recorded in amphibians during summer. In males, the highest T level was observed in fishes during summer, while the lowest one was found in amphibians in the same season. According to the results, significant changes in levels of steroid hormones, as well as thyroids in the selected animals from different classes were closely correlated to the length of the day and seasons.