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Brown rats were trapped each month for 12 months on two pig farms in Co. Kildare, Ireland. Reproduction occurred throughout the year with peaks in spring and autumn as evidenced by juvenile emergence and the pattern of adult fecundity. Fecund males and reproductively active females were those in the heaviest weight groups and with good body condition as indicated by high levels of body fat. Male rats showed signs of delayed sexual maturation and females, though fertile, did not bear litters until relatively late in life (> 300 g). The level of wounding was related to the reproductive status of the animals and males had more wounds than females. The pattern of wounding was probably caused by inter-male conflict for access to breeding partners. Free-living populations of brown rats maintain population stability through controls on the rate of reproduction mediated by a complex social structure.