The effect of aflatoxin B1 (AFB1) on the glutathione S-transferase activity (GST) and on non-protein thiol levels of different tissues was studied in adult male Wistar rats. Animals received a single dose of the toxin (100 or 500 micrograms/kg body wt., p.o.), and were studied 6 or 24 h after administration. GST was determined in liver, renal cortex, duodenum, jejunum-ileum and distal ileum, using 3 substrates: 1-chloro-2,4-dinitrobenzene (CDNB), trans-4-phenyl-3-buten-2-one (PBO) and 1,2-epoxyethylbenzene (STOX). The non-protein thiol content of all tissues tested increased with the lowest dose at 6 h, returning to normal values at 24 h, while the higher dose produced a significant decrease in reduced thiol levels at 6 h, returning to normal values at 24 h. AFB1 administration induced, independently of dose and tissue, total GST (CDNB) and epoxide-transferase activity (STOX) while A--C-type transferases (PBO) were inhibited. Almost all activities returned to normal values at 24 h. In cases of enzyme induction there was in general an increase in Vmax and a decrease in apparent Km. The opposite was seen in cases of inhibition. In conclusion, the results provide evidence that extrahepatic GST could be important in the overall process of detoxification of AFB1. The behavior seen in hepatic and extrahepatic tissues revealed the functions of catalysis (B-type transferases) and covalent bond formation, as well as inactivation by probable AFB1 metabolites (A--C-type transferases).