To characterize the tissue-specific distribution and developmentally regulated expression of M and B creatine kinase mRNA in rats, total cellular RNA was isolated from adult rat tissues and from skeletal muscle, heart, brain and intestine at selected stages of development. Northern blots were prepared and hybridized with M and B subunit-specific probes derived from the 3'-untranslated region. M creatine kinase mRNA was expressed abundantly in heart and skeletal muscle, and less abundantly in lung. B creatine kinase mRNA was found in all tissues examined except liver and was abundant in brain, heart and intestine. The developmentally regulated expression of M and B creatine kinase mRNA was determined in skeletal muscle, heart, brain and intestine. The developmental program of B creatine kinase mRNA was different for each tissue examined. During development, M creatine kinase mRNA was up-regulated in both heart and skeletal muscle with a different regulatory program. This resulted in replacement of B mRNA by M mRNA as the predominant species at an earlier developmental stage in heart when compared to skeletal muscle.