The effect of endothelin (ET) 1 on intracellular Ca²⁺ transients in cultured rat hepatic stellate cells (HSCs) during transformation was studied by use of single‐cell fluorescence. Regardless of the duration of HSC culture, ET‐1 caused a BQ‐123–sensitive but IRL‐1038–insensitive elevation of [Ca²⁺]_i, indicating the involvement of ET_A but not ET_B receptors. HSCs in early culture (“quiescent HSCs”) were mildly responsive to ET‐1: the ET‐1 concentration required to obtain a [Ca²⁺]_i transient in 50% of the cells (RC₅₀) was 7 nmol/L, and all cells responded to ET‐1 concentrations above 40 nmol/L. With culture time, α–smooth muscle actin (α‐SMA) expression increased, as did the ET‐1 sensitivity of cells, resulting in a shift of the RC₅₀ value from 7 nmol/L to 13 pmol/L within 8 days. Independent of the duration of culture, ET‐1 sensitivity was higher in α‐SMA–expressing cells. On the other hand, sensitivity of HSCs to produce a [Ca²⁺]_i response to extracellular uridin 5′‐triphosphate (UTP) or phenylephrine did not change during the activation process. There was no difference between quiescent and activated HSCs with respect to the sharing of intracellular Ca²⁺ stores, which could be mobilized by ET‐1, UTP, and phenylephrine, respectively. The data suggest three conclusions. (1) A marked increase in ET‐1 sensitivity of HSCs during the activation process suggests a potentiation of autocrine/paracrine stimulation. (2) HSCs are susceptible to α‐adrenergic and purinergic stimulation, but sensitivity to phenylephrine and UTP is not affected during the transformation process. (3) The ET‐1–mobilizable Ca²⁺ store is contained in and is smaller than the Ca²⁺ pool, which is mobilized by phenylephrine or UTP.