Given the potential significance of enzyme activities as a link between internal metabolic pathways and environmental nutrients, we investigated the relationships of nitrate reductase (NR) and alkaline phosphatase (AP) with primary production and inorganic nutrients in South Shetland Islands, Antarctica. Enzymatic activities of the phytoplankton (0.7-210 µm), primary productivity, autotrophic biomass and inorganic nutrients were studied in the upper 100 m depth at nine stations during a cruise in the northwestern area of South Shetland Islands (Antarctica), during late austral spring (December 2000). NR activities fluctuated between 0 and 42.8 nmol L-1 h-1 (mean = 10.08 nmol L-1 h-1, SD = 10.42 nmol L-1 h-1), AP activities between 0.81 and 5.67 nmol L-1 h-1 (mean = 2.68 nmol L-1 h-1, SD = 0.95 nmol L-1 h-1). Stations with primary productivity (PP) and chlorophyll a greater than 2 mg C m-3 h-1 and 0.75 mg chlorophyll a L-1, respectively, presented higher enzymatic activities of nitrate reductase, alkaline phosphatase than those stations characterized by primary productivity and chlorophyll a less than 2 mg C m-3 h-1 and 0.17 mg chlorophyll a L-1, respectively. The AP specific activity was negatively correlated with orthophosphate concentrations lower than 2.0 µM, which indicates that the microplankton were under phosphate deficient environment condition. Our results indicated that NR specific activity was positively associated with autotrophic biomass and primary productivity estimates, giving evidence of the use of nitrate by phytoplankton as external nitrogen source in surface waters. In addition, high NR activities were positively correlated with NO3-, suggesting the occurrence of nitrate respiration in the well oxygenated surface waters of Antarctica