Mitochondria from mucosa of rat small intestine were separated (0-11% gradient of ficoll) into fractions, differing by the content of protein, DNA and cytochrome a+a3 as well as by the respiratory activity and the activity of succinate- and NADH-dehydrogenases. Content of DNA and cytochrome a+c was shown to be increased in "heavy" mitochondrial fractions. Mitochondria od "midle" fractions possessed the highest functional activity. The data obtained suggest that the mitochondrial preparations are not contaminated by other cytoplasmic structures; they were also not impaired during isolation and fractionation. The activity of RNA and DNA synthesis in vivo and distribution of labelled mitochondrial RNA and DNA in the fractions were studied. The possible relation of mitochondrial heterogeneity to, their biogenesis is discussed.