We have isolated a novel member of putative serine/threonine kinase from a rat heart cDNA library using polymerase chain reaction methods. The novel kinase is transcribed as 2.6 kb mRNA encoding for a protein of 629 amino acids with the C-terminal non-catalytic portion. Amino acids analysis revealed that the N-terminal catalytic domain is 87% identical to the male-germ cell associated kinase (MAK), a cdc2-related serine/threonine kinase found to promote meiosis during spermatogenesis. Therefore, we designated this novel kinase as the MAK-related kinase (MRK). MRK protein, with a molecular weight of 66 kD, was shown to phosphorylate itself and the exogenous substrates, histone H1 and myelin basic protein. In addition, phosphoamino acid analysis confirmed the serine/threonine-specific protein kinase activity of MRK. Although MRK was ubiquitous in adult rat tissues, the expression of MRK protein in embryos was restricted primarily to embryonic myocardium during early organogenesis. This finding suggests that MRK may be a participant in cardiac development.