Intrarenal distribution of blood flow was assessed with radioactive albumin microaggregates (MA) in three cortical zones of the dog kidney. The experimentally obtained zonal fractions of total renal blood flow were compared with predicted zonal blood flow fractions obtained in a mathematical model. The maximal degree of skimming that could possibly occur in a single experiment was estimated. The analysis showed that local blood flow in the inner cortical zone was maximally underestimated by 17% because of skimming of MA, and in the outer cortical zone the blood flow was maximally overestimated by 13% with the method of radioactive MA uptake. Renal uptake of 201Tl was measured simultaneously in exactly the same locations. Paired measurements of intrarenal blood flow distribution by MA and Tl uptake methodologies showed that local blood flow assessed with MA in the inner cortical zone was significantly lower than that obtained with 201Tl and that a higher blood flow rate was obtained in the outer cortical zone with MA compared with 201Tl. This disparity could be accounted for by the effect of skimming of MA as predicted by the model.