Eighteen green iguanas (Iguana iguana) were randomly assigned to three groups: control group (sham orchidectomy); group I (orchidectomy before the breeding season); and group II (orchidectomy during the breeding season). Circulating testosterone concentrations were determined using a radioimmunoassay and correlated with aggressive behavior scores. Monthly mean (± SD) testosterone concentrations and aggression scores varied with group and season. The highest testosterone concentrations were present in the control group (mean ± SD: 11.6 ± 2.8) during the breeding season (November through January). Likewise, the highest aggression scores were present in the control group (11.6 ± 9.6) and group II animals (4.7 ± 2.2) during the breeding season (November through January). Elevations in testosterone concentration preceded initial increases in aggressive behavior by approximately 1 month. Orchidectomy significantly decreased plasma testosterone concentrations between groups, with significant differences seen between the control group and group I animals and between the control group and group II animals. During the breeding season, significant differences in aggressive behavior were found between all three groups. The control group was the most aggressive, and group II animals were more aggressive than group I animals. Linear regression analysis showed a significant positive relationship between plasma testosterone concentrations and aggressive behavior in the control group (P = 0.008).