We studied the reproductive cycle in males of the cricetid rodent species Oligoryzomys flavescens (Waterhouse, 1837), collected in northern Uruguay. From February 2001 to February 2002, specimens were captured in grasslands and exotic forests. Determination of the reproductive condition was based on external characteristics of gonads as well as histological and histometric analysis. The testicular volume was measured and testes position (whether scrotal or intra-abdominal) was verified by dissection. Correlations between these anatomical features and some environmental factors that included temperature, precipitation and photoperiod, were analyzed using linear regression analysis. The annual reproductive cycle of O. flavescens was characterized by a period of quiescence during winter months (June to August) and a main period of reproductive activity from spring to early fall (October to April). Spermatogenic activity ceased completely in winter and was gradually restored in early spring (September). Reproductively active animals had larger testes and thicker seminiferous tubules than non-reproductive individuals. In addition the epididymides of the former were filled with sperm. Ambient temperature and photoperiod appear to be the main environmental factors that are directly correlated with the reproductive cycle. Finally, in spite of its wide use, it is shown that testes position is an inaccurate predictor of reproductive activity since many individuals showing full spermatogenic activity had intra-abdominal testes. Testicular volume and histological analysis are more appropriate in determining the male reproductive condition of the species. ©SAREM, 2008.