Inorganic nitrogen supply is an important factor limiting primary productivity in Lake Tahoe, Californai—Nevada. To estimate the influence of a watershed ecosystem on nitrogen input to the lake, samples of precipitation, soil water and stream water were collected and analyzed for NO_3—N, NH_4—N and, with less intensity, dissolved organic nitrogen. Factors controlling nitrification in selected plots were studied using a soil perfusion technique. Inorganic nitrogen input by precipitation to the watershed is 1—2 kg° ha(—1)°yr(—1). Output of inorganic nitrogen from the watershed to the lake is 10 to 33% of the precipitation input. Inorganic nitrogen is removed from snowmelt water almost completely by well—developed conifer stands, but less efficiently by poorly developed or disturbed stands. Alder stands release NO_3—N to soil water especially in fall and early winter. Nitrification does occur in some coniferous forest soils in the watershed and is closely associated with NO_3—N release. The occurrence of significant nitrification in the watershed is related more to the C:N ratio and stand basal area than to either pH or release of toxic substances by decaying litter.