The induction of peroxisome proliferation was examined in rat liver after administration of equal concentrations (1 mmol/kg body weight) of 1,10-bis(carboxymethylthiodecane) (BCMTD), 1-mono(carboxymethylthiotetradecane) (CMTTD), 1-mono(carboxymethylthiooctane) (CMTO), 1-mono(carboxyethylthiotetradecane) (CETTD), palmitic acid and hexadecanedioic acid (HDDA). BCMTD, a non-beta-oxidizable and non-omega-oxidizable sulphur-substituted fatty acid analogue was considerably more potent than CMTTD (only non-beta-oxidizable) in inducing enlargement of the liver and increasing peroxisomal activities (monitored by peroxisomal beta-oxidation, palmitoyl-CoA hydrolase and catalase activities). Morphometric analysis of randomly selected hepatocytes revealed that BCMTD and CMTTD treatment increased the number and size of peroxisomes and the relative volume fraction of the peroxisomes. All these cellular responses were more marked with BCMTD than compared with CMTTD. CMTO, a non-beta-oxidizable fatty acid analogue containing a lower hydrophobic alkyl-end than CMTTD and CETTD (a beta-oxidizable fatty acid analogue), showed a slight increase (1.4-1.8-fold) of peroxisomal beta-oxidation and caused marginally morphological changes of peroxisomes compared with CMTTD and BCMTD. The most striking effect of the alkylthiopropionic acid (CETTD) was an enhancement of the hepatic triacylglycerol level. Palmitic acid and hexadecanedioic acid only marginally affected the peroxisomal activities, but no morphological changes of peroxisomes and fat droplets were observed. The presented data strongly suggest that a minimal structural requirement for a peroxisome proliferator may be (1) a carboxylic acid group linked to (2) a hydrophobic backbone which (3) cannot be beta-oxidized i.e., the fatty acid analogues have a sulphur atom in the beta-position. It is also conceivable that blockage for omega-oxidation may potentiate the peroxisome-proliferating activities in as much as BCMTD was more potent than CMTTD. Two mitochondrial marker enzymes, carnitine palmitoyltransferase and succinate phenazine methosulphate oxidoreductase were differently affected after administration of the investigated compounds. Furthermore, BCMTD and CMTTD as well as HDDA treatments increased the number of mitochondria, but the mitochondria tended to be smaller. The overall results presented here indicate that the structural requirements for proliferation of mitochondria are not identical to those for proliferation of peroxisomes.