Supplementary feeding experiments were used to directly test the hypotheses that populations of two species of Australian desert rodents (Pseudomys hermannsburgensis and Notomys alexis) are food limited. Addition of sunflower seeds to a series of trap plots resulted in an increase in the captures of P. hermannsburgensis. Captures of P. hermannsburgensis were related directly to the amount of supplementary food provided and the amount of food consumed. Supplementary feeding was not, however, able to reverse a population decline, yet it did slow the overall rate of the decline in P. hermannsburgensis. Notomys alexis showed an increase in mean body mass in response to supplementary feeding, but population numbers did not change. Neither species showed a long-term response to the dispersion of supplementary food (clumped vs spread). The combination of pattern analyses and experimental manipulations suggests that the population of P. hermannsburgensis is limited by the availability of food, whereas the population of N. alexis is not. This contrasts with natural patterns of increase, which suggest similar mechanisms are involved in both species, highlighting the need for experimental evidence to support correlative studies.