Thyroid hormones are involved in various thyroid activities in higher and lower vertebrates however, the activities are not clearly known. Studies on seasonal changes in glandular and circulating levels of thyroid hormones in different species have revealed varying patterns of changes. Apparent circadian rhythms in plasma and thyroid contents of T4 and T3 have been reported. However, all these findings are limited to temperate zone species and no corresponding information is available on any sub-tropical anuran. Therefore, the purpose of the present study was to estimate seasonal changes in thyroid activity in relation to reproductive events in a sub-tropical male toad, Bufo melanostictus. Changes in circadian level of T3 and T4 were monitored to correlate data with daily and seasonal metabolic events in this species. Thyroid histology was studied microscopically; radioimmunoassay of T3 and T4 was studied. In the present study, it was observed that thyroid activity in male toad was maximum during February and minimum during August. The results of the present study on Bufo melanostictus also showed that circulating levels of thyroid hormones, T3 and T4, both are minimum during June/July. Progressive increase in plasma T3 and T4 levels of Bufo melanostictus was associated with progressive increase in gonado somatic index of male toad between August and December. Plasma T3 and T4 both had slight depression during December-January which was followed by continued increase in T4 in males until March/April. High level of circulating T3 and T4 in association with maximum activity of thyroid follicles probably indicates to the fact that high synthesis and release of thyroid hormones during this period is related to emergence of toads from hibernation.