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Short-lived small rodents require environmental cues to synchronize their breeding with optimal environmental conditions. In the absence of such cues animals would waste a large portion of their reproductive effort when environmental conditions were unfavorable. Rowan (1938) noted that endocrine systems regulating mammalian reproduction responded to the environmental photoperiodic cues. Bodenheimer (1946) observed that major outbreaks of voles in Palestine could not be correlated with environmental factors such as climate, photoperiod, volume of food resources, or population density, acting either alone or in combination. From those studies he suggested that vole outbreaks were associated with an unknown factor or factors in the food supply. Research on Rocky Mountain species of microtine rodents have demonstrated that they respond to both inhibitory and stimulatory plant compounds which result in their reproduction only during environmentally favorable periods (Negus et al., 1977). These compounds and their effects have been characterized principally in female montane voles (Microtus montanus). Berger et al. (1977) reported on the effect of paracoumaric acid (PCA) and ferulic acid (FA), which were isolated from senescent grass, on montane voles and described decreased uterine weight, cessation of follicular development and a cessation of breeding in these voles. Further the action of green vegetation on reproduction was characterized as increased uterine weight, increased follicular development, increased litter size and early attainment of sexual maturity in young voles (Pinter and Negus, 1965; Negus and Pinter, 1966; Pinter, 1968; Negus and Berger, 1971; Berger and Negus, 1974; Negus and Berger, 1977). Sanders et al. (1981) characterized, isolated and identified the active compound from grass as 6-MBOA which is a non-estrogenic compound naturally occurring in most vegetation as one responsible for the reproductive effects reported by others. He demonstrated a dose response for the compound by assaying the uterine weight response of subadult montane voles and determined that maximal uterine responses occur at concentrations the animal normally encounters as a result of eating fresh grass. He additionally reported that ovariectomized voles do not exhibit a uterine weight response and that in laboratory mice increased ovarian weights occur as a result of 6-MBOA stimulation. Berger et al. (1981) reported that in naturally occurring non-reproductive wild populations of montane voles given oat seeds treated with 6-MBOA respond with increased reproductive organ weights and begin to produce litters while oat-supplemented populations did not respond. Both of their methods (intraperitioneal injected or food coated) yielded similar results in response to 6-MBOA. This report will characterize the effect of 6-MBOA on both meadow voles (Microtus pennsylvanicus and pine voles (Microtus pinetorum). Intraperitoneal injection of voles at two developmental ages were carried out to determine if 6-MBOA's effects varied with animal age.