Methanolic extracts from the leaves of Rosmarinus officinalis (rosemary) harvested from different locations of Turkey at four different times of the year were analyzed by HPLC, and their radical scavenging capacities and antioxidant activities were studied by various assays. The amounts of carnosol, carnosic acid and rosmarinic acid, active constituents of rosemary, varied in different geographical regions of growth, and also showed a seasonal variation. The levels of the constituents were higher in the warm months of June 2004 and September 2004. The antioxidant activities of 12 extracts were determined by in vitro DPPH radical scavenging activity, by Trolox equivalent antioxidant capacity (TEAC), and by reversing H2O2-induced erythrocyte membrane lipid peroxidation (EMLP). The two antioxidant enzyme activities of human erythrocyte, namely superoxide dismutase (SOD) and catalase (CAT), after in vitro incubation with the extracts, were also examined in order to see whether the observed effects are related to altered enzymatic efficiency. The resulting values were correlated with active metabolite and total phenol contents of the extracts. The results indicated that the plants harvested in September possessing higher levels of active constituent had superior antioxidant capacities compared to the ones collected at other times. With respect to the location, plants harvested from the Izmir region had lower total phenol and active constituent levels resulting in poorer antioxidant activity.