A study of the relationship between the mating behaviour of the vole and the induction of ovulation and CL function is described. A single intromission or an injection of LH-RF constitute stimuli which induce ovulation, but normally give rise to CL that degenerate soon after formation. More prolonged mating, or mechanical stimulation of the vagina and cervix given after a separate ovulatory stimulus, result in the maintenance of the CL. Mechanical genital stimulation is effective in inducing CL maintenance when given up to 48 hr after an LH-RF injection. Similarities, therefore, are apparent between the vole, an induced ovulator, and spontaneous ovulators such as the rat, mouse and hamster.