Oxydoras sifontesi has an annual gonadal cycle closely linked to changes in the environmental conditions of Venezuelan floodplain. The gonadosomatic index (GSI) and plasma levels of steroid hormones were studied from fish samples collected bimonthly from the Portuguesa River (Portuguesa State, Venezuela) between May 2004 and April 2005. Blood samples from the caudal vasculature were obtained in order to determine sexual hormones concentration. Plasma concentrations of estradiol (E2) in female and testosterone (T) in male were determined by ELISA commercial kits. There was a significant variation in the GSI and sexual steroid hormones during the reproductive cycle of Oxydoras sifontesi. Both sexes showed that GSI and E2 or GSI and T values were higher during the prespawning period (April-May) than in the other periods. The concentration of plasma E2 and T was lower during the preparatory period (March), when GSI were also low.