In the present study the effects of adrenal corticoids, both natural and synthetic, namely cortisol and dexamethasone respectively, was observed on the thyroid gland cell morphology and proliferation in neonatal male chicks (Gallus domesticus). Cortisol was injected at a dose of 4 mg/100 g body weight and dexamethasone at a dose of 1 mg/100 g b.w. subcutaneously daily for fifteen consecutive days. The control birds were similarly injected with normal saline at a daily dose of 0.2 ml per bird for the same time period. The results indicated that both cortisol and dexamethasone caused a significant decrease in thyrofollicular cell height. On the contrary, a significant increase in the ratio of the follicular diameter to the number of nuclei per follicle i.e. D/N value was observed in both cortisol and dexamethasone treated chicks. It was also observed that both cortisol and dexamethasone induced suppression of mitotic activity, as evidenced from a significant decrease in mitotic percentage compared with the control chicks. The present authors' studies thus indicate that adrenal corticoids act as inhibitory modulators of thyroid follicular activity as regards karyomorphology and cell proliferation.