Mechanical stimulation of mesangial cells enhances expression of early response protooncogenes in an extracellular Ca(2+)-dependent manner. This has lead investigators to postulate that a mechanosensitive Ca2+ channel may exist in these cells. At the present time, however, very little is known about single channel Ca2+ currents in mesangial cells. To examine this question, cell-attached patches of rat mesangial cells in early passages were studied with 100 mM BaCl2 patch-clamp pipets. Our studies identified a 21 pS, Co(2+)-sensitive channel that responded with an increase in open probability to mechanical stretch, applied through the patch pipet. The channel was also activated by a reduction in extracellular osmolarity. In both instances, channel responses underwent spontaneous adaptation despite continuous stimulation. This Ca2+ channel exhibited electrophysiological and kinetic properties that distinguished it from other cation-selective channels previously reported by us and other investigators on rat mesangial cell membranes.