Aims - To understand the mechanism for regulation of intraocular pressure, human trabecular cells were examined to determine whether they could respond to the change in hydraulic pressure. Methods - Human trabecular cells were cultured from trabeculum tissue fragments excised during trabeculectomy in four eyes of three patients with primary open angle glaucoma and exposed to the change of hydraulic pressure in a tissue culture flask connected to a glass syringe. The pressure was exerted by automatic infusion of the piston of the syringe and monitored by a pressure gauge. The intracellular calcium concentration was measured in real time with a calcium binding fluorescent dye, fluo-3. Results - A small number (about 10%) of cells appearing morphologically to be trabecular cells showed transient elevations or oscillations of the intracellular calcium concentration in response to the elevation of hydraulic pressure to 20-30 mm Hg, indicating that a part of the human trabecular cells could sense the change in hydraulic pressure. Conclusion - Some cells in the human trabecular tissue seem to sense the change in intraocular pressure and might play a role in its regulation.