In a replicated experiment, nonbreeding winter populations of Microtus montanus were given supplements of rolled oats coated with 6-methoxybenzoxazolinone, a naturally occurring plant derivative. After 3 weeks of this feeding regime, samples from the populations demonstrated a high incidence of pregnancy in females and testicular hypertrophy in males. Control populations receiving rolled oats coated only with the solvent showed no reproductive activity. These results demonstrate that the presence of 6-methoxybenzoxazolinone in the plant food resource acts as the ultimate cue to trigger reproductive effort in Microtus montanus.