ABSTRACT To investigate the nuclear signalling pathway induced by endothelin (ET) isopeptides, we have established permanent Chinese hamster ovary (CHO) cell lines, CHO-ETA/fos-lacZ and CHO-ETB/fos-lacZ, that produce both a c-fos-β-galactosidase fusion protein and either the type A or the type B human ET receptor. These cell lines permitted a colorimetric measurement of c-fos expression, which was induced by the signal transduction system with ET receptors and ET isopeptides. We found that the ET-1-dependent c-fos expression was so efficient that it could respond to low concentrations (even a physiological concentration) of ET-1. For example, CHO-ETA/fos-lacZ and CHO-ETB/fos-lacZ responded to ET concentrations of 5×10−9m and 5×10−13m respectively. Using this highly sensitive system, the H-7 sensitive protein kinase was found to be involved in signal transduction mediated by ETA, and also partly in the ETB-mediated pathway. These lines of evidence suggest that c-fos expression occurs through at least two different pathways, depending on the concentration of ET in plasma.