Lamb: ewe ratio data spanning the years 1953-82 for mountain sheep (Ovis canadensis) in the Santa Rosa Mountains of southern California were analyzed by multiple regression to investigate hypothesized controlling factors. Precipitation during November, January, and February as independent variables each showed a significant positive influence on recruitment rate, in accord with expectations for this desert environment. Between 1962 and 1976, an unidentified 4th factor (probably rising population density) caused a slow decline in recruitment ratio. This ratio plummeted in 1977 and averaged about 25% of the ratio predicted from precipitation alone for 1977-82, a period coincident with a suspected disease epizootic. A similar period of depressed lamb recruitment was found for the 1953-61 period, suggesting the possibility of a recurrent phenomenon. Data needs and various hypotheses concerning factors potentially underlying the observed and future population dynamics are discussed. J. WILDL. MANAGE. 51(1):86-98 Multiple factors potentially underlie the dynamics of wild ungulate populations. The partitioning of the relative magnitude of the influence of different factors in different species and ecological settings is one of the primary objectives of research related to wildlife management. This understanding is crucial if management activities are to be focussed on factors whose manipulation will produce a population response (Leopold 1933). California currently has mountain sheep populations in approximately 50 mountain ranges, mostly in desert regions (Weaver 1975, 1982). Adequate data existing for a small number of these populations indicate recent dynamics varying from rapid population increases to disappearance (DeForge et al. 1981; Weaver 1982; Bleich 1983; Holl and Bleich 1983; Wehausen 1983a,b). Weaver (1975) considered the mountain sheep population of the Santa Rosa Mountains to be one of the 2 largest populations in California. Recently it has come to the forefront as a major population suggested to be declining rapidly due to excessive lamb mortality that began in 1977 (DeForge and Scott 1982). Blood samples from mountain sheep in this range have yielded titers to 4 viral diseases: parainfluenza-3 (PI-3), bluetongue (BT), epizootic hemorrhagic disease (EHD), and contagious ecthyma (CE) (DeForge et al. 1982; Turner and Payson 1982a,b). PI-3 and BT have been suggested as factors predisposing lambs to fatal bacterial pneumonia. This paper attempts to quantify the influence of the purported disease epizootic and other variables on lamb recruitment through statistical analyses of lamb:ewe ratio data spanning 30 years. In addition to the expected depressing effects of diseases on lamb recruitment, it was hypothesized that precipitation and population density also should influence recruitment rate through their influence on nutrition. The Santa Rosa Mountains are a desert ecosystem of low and variable precipitation (Ryan 1968). Consequently, variance in precipitation was expected to strongly affect nutrition and survival of lambs through its influence on vegetative growth (Monson 1960). Work in the nearby Mojave Desert has indicated that the timing of precipitation is of particular importance to subsequent plant growth (Beatley 1974). Specifically, fall precipitation is a crucial determinant of whether annual and perennial forb growth will be present the following spring and strongly influences spring growth of shrubs and perennial grass species. Winter and early spring precipitation also can trigger growth in shrubs and perennial grasses. Turner (1973) found that mountain sheep in the Santa Rosa Mountains consumed a mixture of browse, forb, grass, and cactus species that varied seasonally and yearly with availability of succulent growth. Particularly noteworthy relative to nutrition of lambs was an increase in consumption of forbs and green grasses during spring in years when these were available.