N-methoxycarbonyl-N′-(2-nitro-4-propylthiophenyl) thiourea and N-methoxycarbonyl-N′- (2-nitro-5-propylthiophenyl) thiourea, prodrugs of albendazole, have been synthesized. The biotransformation in rats, after oral administration of these prodrugs to albendazole and albendazole sulphoxide, is described.