Adult male green iguana (Iguana iguana) (n=10) were collected in different seasons in the Brazilian Pantanal and their testes were examined using light and scanning electron microscopy. The annual reproductive cycle involved four stages which alternated between a maximum and a minimum of spermatogenesis. During the cycle, there was an inversely proportional relationship between the development of the seminiferous tubules and the interstitial tissue. In the first stage of the cycle (October-December, inundation season) there was minimum spermatogenesis, with only the initial divisions of spermatogonia, whereas the interstitial tissue and Leydig cells were well developed. This state continued during the second stage (January-March, flood season) with initial germ cell development and a reduction in interstitial tissue. In the third stage (April-June, drainage season) there was maximal germ cell activity with all phases of cell maturation present whereas the interstitial tissue was greatly reduced. Spermiation occurred in the last stage (July-September, dry season) and left large mounts of residual cytoplasm in the seminiferous tubule lumen. The possible social and environmental implications of this cycle are discussed.