Population fluctuations are the result of the combined action of endogenous (feedback structure) and exogenous factors (large- and local-scale climate variables). In this paper, we used a 13-year time series to identify the feedback structure in a population of the brown 4-eyed opossum Metachirus nudicaudatus and to test a hypothesis on the effects of El Niño Southern Oscillation and rainfall using Royama's theoretical framework. Metachirus nudicaudatus was regulated by a strong 1st-order negative feedback, with intraspecific competition for food resources as the probable factor governing the endogenous system. Contrary to our expectations, El Niño did not explain the marsupial dynamics better than 1-year lagged rainfall, that may operate in 2 different manners: as a nonlinear perturbation effect influencing the strength of density dependence (intraspecific competition and intraguild predation) or as a lateral perturbation effect influencing the carrying capacity of the environment.