Juvenile male Microtus montanus were examined for the effects of photoperiod, melatonin, and the naturally occurring reproductive stimulant 6-MBOA on growth and sexual maturation. 6-MBOA, present in sprouting grass, is an important environmental cue used for the initiation of reproduction in natural populations of this species. Long photoperiod (16:8) was stimulatory to body, testes, and seminal vesicle growth, while short photoperiod (8:16) inhibited these parameters. The pineal hormone melatonin, administered via daily afternoon injections (5 micrograms), was also inhibitory to all of the above parameters as well as to serum testosterone. 6-MBOA, administered via injection (0.0001-100 micrograms) or dietary means (0.1 or 1.0 microgram/gm unsifted chow), appeared unable to augment the rate of maturation in long-photoperiod-stimulated animals. When short-photoperiod animals were treated with high doses of the compound (100 micrograms injected or 1.0 micrograms/gm sifted chow), body and gonadal growth was inhibited to a greater extent than when animals were exposed to short photoperiod alone, and serum LH was reduced. Lower doses of the compound had no effect. Melatonin-treated animals experienced less maturational inhibition when simultaneously given a low dose of 6-MBOA-coated chow (1.0 micrograms/gm unsifted chow). A higher dose of 6-MBOA (1.0 micrograms/gm sifted chow) was ineffective in preventing the response to melatonin. These results indicate that 1) male M. montanus utilize photoperiod, rather than 6-MBOA, as a primary environmental cue, 2) high doses of 6-MBOA can be inhibitory under short photoperiod, 3) juvenile male voles are highly sensitive to the inhibitory effects of exogenously administered melatonin, and 4) 6-MBOA can partially prevent the inhibitory effects of melatonin on growth and sexual maturation.