1. The annual profiles of plasma thyroid-stimulating hormone (TSH), thyroxine (T4), luteinizing hormone (LH) and testosterone (T) concentrations in control and ganglionectomized (SCGx) dormice were analyzed to determine whether the pineal gland affects thyroid-gonadal interactions in response to the seasonal influence of environmental factors. 2. Dormice ganglionectomized in September, around the time of prehibernation when hormonal activity is minimal had significantly disturbed annual cycles of plasma T4, LH and T, while the TSH cycle was unchanged. 3. SCGx performed after breeding season (June), only affected the T4 variations, while the T and LH titers were similar to those of controls. 4. We conclude that the annual cycles of T4 and testosterone are controlled by the sympathetic nervous system via the superior cervical ganglion which innervates both the pineal gland and the thyroid. 5. During the start of seasonal gonadal activity, which is strongly dependent on thyroid-gonadal interactions, the inhibitory action of the pineal gland on the neuroendocrine thyroid axis, or most probably a direct inhibition of the thyroid by the sympathetic innervation from SCG, might influence the timing of the reproduction cycle. 6. The lack of thyroid-gonadal interaction at the end of the breeding season suggests that the thyroid disturbance caused by ganglionectomy in June does not cause gonadal perturbation at this time.