Forty female meadow voles (Microtus pennsylvanicus), bred from stock trapped in the wild, showed no regular cycles in their vaginal smear patterns. At autopsy, twenty virgin voles showed no evidence that spontaneous ovulations had occurred. Twenty recently mated females had fresh corpora lutea in their ovaries when killed. The conclusion was drawn that under laboratory conditions this species is an induced ovulator, coitus providing the stimulus for ovulation. A leucocytic invasion of a previously cornified or nucleated smear pattern is an indicator of ovulation in the meadow vole.