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The essential role of the anterior pituitary gland in the development and the function of the avian gonad has been recognized for more than three decades. However, little is known concerning annual variation of gonadotropic function in species with marked gonadal cycles. The only extensive information available appears to be that of Greeley and Meyer (1953) who investigated the gonadotropic activity of the anterior pituitary in relationship to the annual testicular cycle in Ring-necked Pheasants (Phasianus colchicus) held in captivity under semi-natural conditions. In order to extend our understanding of the regulation of the hypothalamo-hypophysio-gonadal axis in birds under natural conditions, we have investigated the annual cycle of pituitary gonadotropic potency in relation to gonadal cycles in free populations of the White-crowned Sparrow, Zonotrichia leucophrys gambelii Nuttall. Such data are also indispensable in the interpretation of the results of experiments conducted under controlled conditions. The annual testicular cycle of Whitecrowned Sparrows has been investigated by Blanchard and Erickson (1949), by Oakeson (1954), and by Oakeson and Lilley (1960). Our data extend these investigations, and describe also the annual ovarian cycle.