Day 9 rat embryos were exposed to 1,4-dihydropyridine calcium channel blockers; nifedipine (NIF), nicardipine (NIC) or nitrendipine (NIT), for 48 hr in the whole embryo culture system. There were dose-dependent growth retardation and abnormalities, predominantly in cardiovascular system. The three compounds exhibited very similar pattern of dysmorphogenic effects, but the potency of these compounds were quantitatively different. The incidences of embryos with the abnormalities were 100%, 100% and 85% following either exposure of NIF, NIC or NIT at concentration of 300, 8 and 15 microM, respectively. This study was to investigate whether these blocker-induced embryotoxicity was due to calcium channel blocking properties themselves in the embryos. Day 9 rat embryos were co-exposed to 1,4-dihydropyridine calcium channel agonist, Bay k 8644 (BAY) and each calcium channel blocker under the same culture condition. The retarded embryonic growth induced by 200 or 300 microM of NIF, 8 microM of NIC and 15 microM of NIT nearly of completely ameliorated when embryos were co-exposed with BAY at one-third or half concentration of each calcium channel blocker. Supplementation of BAY reduced the incidence of abnormalities by NIF-, NIC- and NIT-alone. These results suggested that one of mechanisms for embryotoxicity induced by calcium channel blocker was directly related to channel blocking property of the chemicals.