From March to October 2001, the effect of N fertilizer on water contamination with nitrates was evaluated in two coffee (Coffea arabica) plantations shaded with Eucalyptus deglupta in the Pacific southern zone of Pérez Zeledón, Costa Rica. These plantations received 260 kg N ha-1 yr-1 until 2000 but only 120 kg N ha-1 during 2001. Concentrations of N-N03 averaged 0.21 mg l-1 under forests and 1- 1.6 mg l-1 in the coffee farms; values lower than the 2.1 to 4.4 mg l-1 reported in coffee-growing areas in the Central Valley of Costa Rica. This could be due to the greater rainfall and the lower fertilization rate as well as less fertile soils in Pérez Zeledón. In this study, N-N03 concentration of springs did not exceed 5.8 mg l-1 which is lower than the 10 mg l-1 considered by the World Health Organization to represent a health hazard. During the rainy season (June), soil nitrates averaged 130 kg ha-1 in the upper meter of soil. Lysimeters located 1.2 m below the soil surface detected higher mean N-N03 concentration (6.2 mg l-1) than in springs water, suggesting that nitrate adsorption or denitrification possibly occurred in soil layers below 1.2 m depth.