The contractile response of cultured Ito cells to endothelin-1 and substance P was examined. Ito cells were obtained from rat liver by perfusion with collagenase, followed by separation through centrifugal elutriation, and were cultured for 24 hr. The area of the Ito cells was measured after treatment with endothelin-1 or substance P at various concentrations in the culture medium. The area of the cells decreased dose dependently after treatment with endothelin-1 or substance P. The area of Ito cells before addition of interleukin-1 or substance P was defined as 100%. The area of the cells after treatment with endothelin-1 or substance P medium was expressed as the percentage against the area before treatment with endothelin-1 or substance P. The percentage in area after treatment with 200 nmol/L endothelin-1 was as follows: 81% ± 13% at 30 min, 77% ± 15% at 60 min, 87% ± 15% at 120 min and 99% ± 18% at 180 min. The maximal decrease in area occurred at 60 min after treatment. The percentage values for 200 nmol/L substance P were as follows: 88% ± 15% at 10 min, 95% ± 17% at 30 min and 101% ± 17% at 60 min. The maximal decrease in area was noted at 10 min. Thus Ito cells contracted in response to treatment with endothelin-1 or substance P. The mode of the extent and onset of the contraction was different for the two peptides. These findings suggest that Ito cells are involved in the regulation of the hepatic sinusoidal microcirculation. (HEPATOLOGY 1993;18:978-983).