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It has been demonstrated that birds of several passefine species show gonadal recrudescence within six or seven weeks, when given sufficient artificial increases in day length in late fall or in winter. Birds treated in late summer or early fall, however, do not respond. The period between regression of the gonad, normally at the end of the breeding season, and the time the gonad responds to photostimulation, has been designated the refractory period. The experiments reported here were undertaken to determine by systematic study when the refractory period ends in the Slate-colored Junco (]unco hyemalis) and in the White-throated Sparrow (Zonotrichia albicollis)--species in which the natural termination of the refractory period had not been investigated. Secondarily, the effect of treatment with short days in the fall on the duration of the refractory period in these species was studied. It is with a great deal of pleasure that I acknowledge the assistance and helpful criticism of Dr. Albert Wolfson. The research reported here was supported in part by a grant from the Graduate School of Northwestern University.

The Natural Termination of the Refractory Period in the Slate-Colored Junco and in the White-Throated Sparrow