Reproductive biology was studied in captive N. alexis, N. fuscus, N. mitchellii and N, cervinus. All were polyoestrous, the oestrous cycle averaging 7.0-8.0 days in N. alexis and N. mitchellii and 9.0 days in N. fuscus. In N. cervinus there was a considerable variation in length; this species may be easily stressed and less adaptable to captivity. Gestation in the non-lactating animal lasted 32, 34-37 and 38-43 days in N. alexis, N. mitchellii and N, cervinus respectively; all three had post-partum oestrus and mating. Lactation appeared to delay implantation in N, cervinus and probably in N. mitchellii, but not in N. alexis; it lasted 3-4 weeks, during which time the young clung tenaciously to the teats. Oestrus was not always suspended during lactation in N. alexis, and conception may take place during suckling in this species. Interspecific variations in this pattern of reproduction are discussed, and the information compared with data from other Australian Muridae.