SUMMARY: The sensitivity of Bufo bufo to chorionic gonadotrophin (C.G.) throughout the year, before and after 4–6 months of enforced hibernation in a refrigerator, is recorded. The sensitivity of newly captured animals was found to show a small seasonal variation, and was highest in those caught in the spring, the normal breeding season. Among animals subjected to between 4 and 6 months' enforced hibernation sensitivity was increased in those captured in the spring and summer. No significant change was detected in the sensitivity of animals caught in autumn. It is suggested that short periods of enforced hibernation of 1 to 2 months may significantly increase the sensitivity of B. bufo to C.G. This appears to be true of animals caught in the spring, and further work will have to show whether it also applies to animals caught at other times of the year.