Knowledge of reproduction in the river otter, Lutra canadensis, and of the factors which control the reproductive cycle, is limited. The reproductive biology of river otters in the marine environment of British Columbia, and some of the factors which may influence their reproductive cycle, in particular, the effect of photoperiod, were examined. The changes that occurred in the anatomy, histology and endocrinology of the annual reproductive cycle were determined in wild (226 male, 146 female) and captive (9 male, 5 female) otters. Reproductive cycles were similar in both groups. As in the fresh-water environment, coastal otters are seasonal breeders with a prolonged, obligatory period of embryonic diapause prior to implantation. Changes were similar to those reported in other mustelids. In males, development of the seminiferous tubules was divided into a series of nine distinct phases used to quantify the degree of reproductive activity present. Testosterone concentrations were highly correlated to changes in testicular activity, and fell into the low end of the range of concentrations seen in mustelids. Vaginal smear types were found to identify correctly the reproductive state of female otters. Estradiol concentrations of captive otters were highly variable, but the values obtained and the pattern of secretion were similar to those of other carnivores. Although estradiol levels of captive otters were reduced in the final two years of the study, normal ovarian development occurred. During pregnancy, progesterone concentrations corresponded to changes observed in luteal cell cytology. I present evidence suggesting that female otters are monestrous, induced ovulators. The timing of reproductive events of otters in the marine environment was similar to those of otters inhabiting fresh water at comparable latitudes. Reproductive activity of males began in the late autumn and adults were capable of breeding from late January through mid-May. Females exhibited estrus from late February through mid-May, with the majority of matings probably occurring during March and April. After a nine to eleven month period of embryonic diapause, implantation occurred the subsequent February. Births occurred after an estimated two month gestation and parturient 1 otters mated during lactation. Male otters were sexually mature at the end of their second year. Only 55% of females bred at age class 2 and sexual maturity was delayed until the following year in the remainder of the females. Pregnancy rates were high among females over 3 years of age and ovulation rates were similar to those of other studies. To determine the influence of photoperiod on the timing of reproductive events, captive otters were exposed to increased photoperiods. With one exception, spermatogenesis and estrus occurred earlier under experimental conditions. My results suggest that the termination of embryonic diapause is also under photoperiodic control. Variations in temperature or food were not associated with changes in reproductive condition. This study suggests that photoperiod probably controls the timing of the reproductive cycle of otters in British Columbia. / Science, Faculty of / Zoology, Department of / Graduate