The reproduction of the possum, Trichosurus vulpecula (Kerr), was investigated on Banks Peninsula, Canterbury, from 1963 to 1965. The majority of births occur in the autumn, but there is a minor breeding season in the spring. After undergoing an anoestrous period during the summer most females enter oestrus around April. A small proportion of females give birth to a second young during September or October. Reproductive periodicity was also found to occur in the male possum. Although spermatogenesis continues throughout the year in the sexually mature animal there is a marked change in the size of the prostatic urethra at particular periods of the year which correspond to those in which the majority of females are undergoing oestrus. The mean weight of the prostate shows a sixfold increase during the autumn. However only very small changes take place in the size of the testis and epididymis. Changes in size and activity of the prostate correspond with marked changes in the interstitial tissue of the testis. Leydig cells, responsible for the secretion of testosterone, show a great increase in number in animals in which the prostate is enlarged. Associated with the growth of the prostate are changes in the behaviour of the possum. The body weight and fat deposits of the male possum drop markedly at the time of the autumn breeding season.