Fumonisins, mycotoxins produced by Fusarium moniliforme and a number of other fungi, are potent inhibitors of the sphinganine-N-acyltransferase, a key enzyme of sphingolipid biosynthesis, and cause neuronal degeneration, liver and renal toxicity, cancer and other injury to animals. In this study we investigated the effect of fumonisin B1 on the sphingolipids of developing chick embryos. After yolk sac injection of fumonisin B1 a concentration and time dependent increase of the sphinganine-over-sphingosine ratio of the embryos could be demonstrated. Studies were done to evaluate the effect of fumonisin B1 on the glycophingolipid pattern of the chick embryos. In the presence of 72 micrograms fumonisin B1 per egg the incorporation of [14C]galactose and of [14C]serine into embryonic glycosphingolipids was reduced by about 70%, although the mass of glycosphingolipids was not affected by the toxin. However, a reduction of the wet weight of the treated embryos was observed. Additionally, histological examinations of whole embryo sections of control and fumonisin B1 treated embryos are presented. Fumonisin B1 caused haemorrhages under the skin as well as in the liver of treated embryos. A close correlation between disruption of sphingoid metabolism and light microscopic detectable tissue lesions could be observed.