Rat liver mitochondrial polyribosomes were isolated free from cytoplasmic ribonucleoprotein contaminations in a number of criteria (sedimentation and buoyant density patterns, ribosomal RNA composition). Heterogeneous poly A containing RNA from mitochondrial polysomes was purified by two-stage cellulose chromatography. This RNA was in vitro labelled with 125I up to specific activity approximately 10(6)-10(7) cts.min-1.microng-1 and used for hybridization experiments with separate complementary strands of mitochondrial DNA and nuclear DNA fragments. The proportions of mitochondrial poly A containing RNA that is complementary to heavy and light strands of mtDNA were respectively 31.5% and 8.3%. Besides, a significant RNA fraction was complementary to unique sequences of nuclear DNA (2-3 copies per haploid genome). The hybrids that were formed possessed a high Tm indicative of a perfect base pairing. A dual intracellular origin of mitochondrial messenger RNA is discussed.