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Plant compounds associated with active plant growth and senescence have been reported to have different effects on microtine reproduction (Berger et al., 1977, 1981). Biological assays of inhibitory compounds showed some effect at pharmacological doses in Microtus pennsylvanicus but not in M. pinetorum (Cranford et al., 1980; Derting and Cranford, 1981). Stimulatory plant compounds contained in active growing wheat have been demonstrated to cause early sexual maturation (Berger et al., 1981) and to induce reproduction in wild populations during non reproductive periods (Negus, 1977; Berger et al., 1981). Bodenheimer (1949) postulated that plants contain compounds which trigger the onset of reproduction in M. guentheri. In M. montanus small amounts of green plants or their extracts supplemented to the normal laboratory diets have caused increased uterine weight, increased numbers of estrus females, increased numbers of young produced, and a return to sexual activity under normally inhibitory conditions (Pinter and Megus, 1965; Negus and Pinter, 1966; Negus and Berger, 1971; Negus and Berger, 1972).