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The simultaneous effects of dietary fiber, temperature, and daylength on the gastrointestinal morphology and physiology in meadow voles (Microtus pennsylvanicus) were investigated. Voles were randomly divided (10 in each group) and placed in 15-day treatment regimes [cold (5 C) and warm (21 C), daylengths long (18L:6D) and short (6L:18D), and diets of high fiber (50% neutral detergent fiber [NDF]) and low fiber (5% NDF) content] followed by a 5-day digestibility trial to determine diet digestibility in relation to food intake. Total mass of individuals, length, wet mass and dry mass of the stomach, small intestine, caecum, large intestine, adrenal glands, liver, and kidneys was evaluated. Data was analyzed by ANOVA using the SAS system. Meadow voles increased food intake but decreased turnover time and digestive efficiency under low ambient temperature condition, short daylength, and/or high dietary fiber. Increased energy demand had a minimal effect on the gastrointestinal tissue mass but resulted in slightly increased length of all tissues except the small intestine. The increased food intake and small changes to gastrointestinal morphology during times of increased energy demands suggest that meadow voles are able to meet their energy needs primarily through increased food intake, and therefore more energetically expensive gastrointestinal changes are minimized. / Master of Science