Amount and oxygen-18 content of rainfall and throughfall have been monitored in a pine forest in Uppsala during summer 1983. It is found that the free throughfall coefficient, C, the canopy reservoir capacity, S, and the critical value of rain, R0, are ≈ 0.5, 2.1 mm and 4.1 mm respectively. The average value of throughfall is about 60% of rainfall. Generally, the throughfall is enriched in oxygen-18 with respect to rainfall, but in very few cases depletion has been observed. The enrichment or depletion in throughfall have been graphically predicted and agree fairly well with the observations. Oxygen-18 values of throughfall have been simulated by a model describing isotopic fractionation in throughfall. Using estimated relative humidity and temperature during the rainfall, the simulations describe quite closely the observed fractionation. It is felt that more accurate estimates of C, S and fractionation in throughfall can be obtained by monitoring rain and throughfall in small fractions of about 0.2 mm or even less.