The photorefractory period of golden hamsters is characterized by the failure of short daylengths (less than 12.5 hours per 24) to induce gonadal regression. Photorefractoriness is terminated by prolonged exposure to long daylengths (greater than 12.5 hours per 24). This study was designed to determine the precise duration of long day (LD 14 hours light/24) exposure necessary to terminate refractoriness. The data indicate that 11 weeks of LD 14:10 terminate refractoriness in nearly every hamster. Exposure for fewer than 11 weeks is less effective whereas exposure for more than 11 weeks is no more effective. The data show that hamsters begin measuring long days to terminate photorefractoriness irrespective of the physiological state of the reproductive system, and, therefore, that photorefractoriness is induced some time during the period of testicular regression (during the first 10-12 weeks of short day exposure).