Karwinskia humboldtiana (Kh) is a poisonous shrub causing a number of accidental intoxications in humans. In previous studies in our laboratory, we reported an increased number of mast cells present in peripheral nerve of Kh intoxicated rats. This finding has not been reported in other organs of intoxicated animals. For this reason, in the present study we searched for mast cells in several organs, identifying mast cell subpopulations on the basis of different histochemical reactivity of their secretory granules. Thus the objective of the present study was to evaluate the presence of mast cells in organs other than peripheral nerve and, to distinguish mast cells by their granule content, applying histochemical reactions. 32 Wistar rats were divided into 4 groups (n=8). For each group, 5 rats were intoxicated with Kh and 3 received water only as a control. Intoxicated rats received 3.5 g/ Kg body weight of dry powder of Kh fruits, fractionated in 5 doses as follows 1.5, 0.5, 0.5, 0.5, 05 on days 0, 3,7,10,14 respectively. Control rats received water only. Each group was killed at different times during paralysis evolution. Samples of liver, kidney, lung and brain, were obtained and processed by routine technique until paraffin embedding. Sections were obtained and stained with toluidine blue, PAS, alcian blue/PAS and alcian blue/safranin. Mast cells infiltrates were observed in peripheral nerve and lung. Mast cells were counted. An increasing number of mast cells were recorded as well as variations in the histochemical pattern of their granules for each organ. These findings suggest a role for mast cells and their secretions in the intoxication with mature fruit of Kh.