The primary objectives of this paper are to define, explore the environmental factors favoring, and discuss hypotheses concerning the endocrine bases of two important arctic breeding strategies that we call spatial and temporal opportunism. We identify several species that display spatial opportunism in the Arctic, and one that displays temporal opportunism. In spatial opportunism, breeding may be highly seasonal but the locality where individuals breed may change from year to year as a result of unpredictable spatial distribution of food, nest site availability, or other factors such as predator abundance. We suggest that flexibility of the transition from migration to settlement distinguishes spatial opportunists from site-faithful migrants. Thus far, data are available for only two hypotheses regarding the endocrine basis of this flexibility. Circulating patterns of testosterone (associated with territory establishment) and corticosterone (associated with migratory activity) appear not to be involved in the regulation of spatial opportunism in white-crowned sparrows (Zonotrichia leucophrys gambelii), but more detailed study is necessary. In temporal opportunism (that may occur simultaneously with spatial opportunism), temporally variable food availability apparently selects for the capacity to initiate reproduction across a wide portion of the year. Tonic activity of the hypothalamo-pituitary-gonad (HPG) axis could provide a mechanism to minimize the delay between discovery of abundant food and onset of nesting in any season. However, the evidence for one arctic temporal opportunist, the white-winged crossbill (Loxia leucoptera), indicates that the HPG axis is not tonically active, but probably switches off (i.e., becomes photorefractory) in autumn, as for other birds breeding at these latitudes. Opportunistic breeding very early in the year (e.g., March near Fairbanks, Alaska) is associated with increased luteinizing hormone secretion, probably in response to a combination of abundant food and social stimuli, after refractoriness dissipates. We have taken the first step of identifying spatial and temporal opportunism as important phenomena in the Arctic, and discussing hypotheses related to endocrine mechanisms. Future research should identify specific environmental cues involved, and elucidate the neuroendocrine and endocrine mechanisms underlying these two reproductive strategies.