SUMMARY The mean blood-sugar level of frogs killed in the field without previous excitement showed no significant variation between April and October inclusive (38±1·42 mg/100 ml.). The resting blood-sugar level at other times of the year, after 2–3 days in captivity, was slightly lower. A marked hyperglycaemic response to excitement could be elicited in the majority of frogs at the spawning season (March) and throughout the summer, while it was only found in April and early May in a few frogs which showed early development of the gonadial fat-bodies. The response was much reduced in September and absent in October. The seasonal occurrence of the hyperglycaemic response paralleled the known cycle of thyroid activity, and a significant correlation was found between the blood-sugar level and thyroid condition in individual excited frogs. A significant response to excitement has been induced in spring, summer and autumn by previous treatment with thyroxine. It is suggested that the thyroid hormone inhibits the destruction of circulating adrenergic compounds. The role of temperature in limiting the hyperglycaemic response is discussed.