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Light and electron microscopical studies were carried out after experimental induced and spontaneous infection with African swine fever virus. The experimental infection was performed in 18 pigs divided into two groups consisting of 9 animals. The pigs of group I were inoculated with virulent isolate E 70, those of group II with attenuated isolate E 75. Two infected pigs of group I and one control animal were killed on days 3, 5 or 7 p.i., two pigs of group II and one control animal were killed on days 9, 11 or 13 p.i. Additionally 19 spontaneous infected and seropositive animals and two seronegative pigs without clinical signs were examined. Infection with African swine fever virus resulted in slight morphological alterations of the intestine. The pathological findings showed a more intense involvement of the large intestine, especially the caecum and colon, than the small intestine, where the ileum was mostly affected. In all three groups edema and vacuolisation could be observed in endothelial cells. In spite of beginning degenerative signs, especially after spontaneous infection, the fenestrated structure of the endothelium was conserved in most of the cases. In animals infected with virulent isolate the vascular lumina contained aggregations of fibrin, which was severe pronounced in the pigs of the other groups. In the area of these alterations disturbance of permeability with extravasation could be found. In all groups single virions or virus aggregates could be identified in concave depressions of the erythrocyte membrane or free within the blood plasma, in some cases enveloped by a plasma-like material.(ABSTRACT TRUNCATED AT 250 WORDS)