Specimens of the sand—dwelling lizard, Uma notata, were collected monthly, primarily in the Algodones Dunes, Imperial County, California, from July 1958 through September 1962. These animals were autopsied to determine the breeding cycle of both sexes. Most individuals reach reproductive maturity during the second summer following hatching. Adult testis volumes change markedly during the year, generally reaching maximum size in May. Males were found as potential breeders from mid—April to mid—September. Females were found with eggs (usually two) in oviducts from mid—May to the late August. Females probably lay more than one clutch per year, based on observations of egg—laying captives. The reproductive period yields slightly from year to year due, at least in part, to the amount of rainfall that occurs the preceding winter. Breeding activity in males and the emergence of juveniles are delayed following dry winters. This is attributed to a series of interactions between organisms. U. notata tests do not become reproductively active if the lizards do not obtain adequate food. Insects that live in low—growing plants (annuals) normally are used for food by these lizards. However, insects that develop on these plants can do so only if sufficient rainfall occurs in winter to produce annuals. On the other hand, the larger plants (perennials) in the dunes acquire sufficient moisture from deep in the sand to flower each year, regardless of winter rains. Consequently, insects that utilize perennials are relatively plentiful every year. Following dry winters, Uma must rely on obtaining sufficient food from these insects, which presumably are more difficult to capture by sand—dwelling lizards. Thus, breeding activity is delayed until the requisite amount of food can be acquired. The reproductive responses of Urosaurus graciosus, a lizard that lives in perennial plants on these dunes, support this explanation.