In an experimental field study, populations of gray-tailed voles (Microtus canicaudus) were exposed to a commercial formulation of vinclozolin, a fungicide effective for disease control on ornamental plants, turf grasses, and fruits and vegetables. Vinclozolin has been shown in laboratory experiments to behave as an androgen antagonist, impairing the reproductive development in males of several species of mammals when exposed in utero. However, when applied to grassland habitat containing populations of gray-tailed voles, no biologically significant impairment was observed in reproductive development of male voles whose mothers were exposed to the treatment while the young were in utero. Reproductive organs were sufficiently developed to result in high reproductive rates and juvenile recruitment in the field with no effects on population growth or demography. Under the conditions of this study, one standard application of Curalan[copyright] fungicide had no measurable reproductive or demographic consequences on gray-tailed voles. However, the results suggest that higher application rates or several successive applications may have negative impacts on reproductive development and demography of wild vole populations. / Graduation date: 1999