Objective To investigate the temporal and spatial expression pattern of the pore-forming α1 subunits of T-type calcium channels during mouse embryonic heart development. Methods Histology and immunohistochemistry of mouse embryonic hearts from embryonic 9.5 days （E9.5） to E18.5 in paraffinembedded were used to analyze the expression of α1G and α1H, two subtypes poreforming α1 subunits of T-type calcium channels. Results α1H proteins were detectable from earlier embryonic days (E10.5) to late embryonic day (E18.5) while α1G proteins from E11.5 to E18.5. Immunohistochemical localization showed that the expression of α1H or α1G throughout myocardium and bundle branches with exception of the endothelium and mesenchyme cells among the trabecular and endocardial cushion re