To understand molecular mechanisms that regulate formation and maintenance of cardiac IKr (rapidly activating component of the delayed rectifier K+ current), we have investigated the spatiotemporal expression pattern of two rat potassium voltage-gated channels, namely subfamily H (eag-related), member2 (KCNH2) (alias name: rERG) and Isk-related family, member2 (KCNE2) (alias name: rMiRP1) during late embryonic development by means of the in situ hybridization technique. KCNE2 is transcribed predominantly in atrial und ventricular myocardium at stages E14.5-E18.5dpc and only a minor signal emerged in the tongue at E16.5dpc. In contrast, KCNH2 transcripts appeared in a less confined pattern with intense signals in atrial and ventricular myocardium, somites, spinal cord, bowel system, central nervous system and thymus at stages E14.5-E18.5dpc. Non-cardiac expression even exceeds the intensity of the cardiac signal, indicating that KCNH2 contributes to K+ currents in non-cardiac tissue as well. Transcription of the rat b-subunit KCNE2 is present in all regions of the fetal myocardium and co-distributes perfectly with transcription of the pore forming a-subunit KCNH2. It seems likely that KCNH2 and KCNE2 are linked to form cardiac IKr channels, associated to cardiogenesis and cardiomyocyte excitability.