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Photo-period and nutrition are important variables affecting reproductive activity and growth in many rodents. Field and laboratory studies indicate that long photo-period (spring-summer) cause Increased growth while short photo-periods (fall-winter) inhibit these processes. In the montane vole (Microtus montanus) recently weaned animals gain weight at a much lower rate under short photo-periods or in total darkness than under long photo-periods (Vaughan et al., 1973; Peterborg, 1978). Adult M. montanus had more offspring and larger mean litter sizes under LD 18:6 than LD 6:18 (Pinter & Negus, 1965). Similarly, long (LD 16:8) or increasing photo-periods stimulated the onset of puberty in M. arvalis, while short (LD7:17) or decreasing photo-periods inhibited the onset of puberty (Lecyk, 1962). Short photo-periods caused reduced spennato-genesis and seminal vesicle weights in male M. arvalis, while long photo-periods induced increased ovulation in females. In contrast, photo-period had no effect on the reproductive rates of M. orchadensis (Marshall & Wilkinson, 1956). Dicrostonyx groenlandicus reared on LD 6:18 grew faster than those on LD 20:4, but the latter group had larger testes (Hasler, 1975). In M. agrestis, long photo-periods stimulated male reproduction and caused greater body weight gain than did short photo-period. Females produced fewer young, had lower ovarian and uterine weights, and fewer, smaller Graafian follicles under short photo-periods as compared to long. However, there was no effect on female body size (dark & Kennedy, 1967; Breed & Clarke, 1970; Baker and Ranson, 1932). Microtus pennsylvanicus juveniles and adults had higher body weights under LD 18:6 than LD 6:18 (Pistole, 1980). M. oregoni reproductive activity is stimulated by long photo-period, but due to fossorial habits this species appears to be less sensitive to light than more terrestrial forms (Cowan & Arsenault, 1958). There have been no reports of winter breeding in Clethrionomys gapperi possibly because of their behavioral avoidance of light during the winter (Evemden & Fuller, 1972).