The ultrastructural changes of single smooth muscle cells of guinea pig taenia coli during contraction evoked by Ca2+ (Ca) and carbachol (CCh) were investigated by transmission and scanning electron microscopy. Cells, prepared in the presence of a high concentration (140 mM) of KCl, without addition of Ca, exhibited smooth surfaced contour. The contraction of these cells were dose dependent upon Ca. With the increase in Ca concentration, the contraction was accompanied with formation of processes (0.15 micron thick and 1 micron long), membrane evaginations of various sizes and shapes and cores densely packed with filaments at the bottoms of all evaginations. Cells, prepared in the presence of 0.18 mM Ca and at a physiological concentration (2.7 mM) of KCl, had two populations: One had shallow invaginations of the cell membrane and the other had spinous processes. Cells of both types were responsive to CCh and the spinous processes disappeared during contraction but evagination did not appear. Differences in ultrastructural changes during contractions between single smooth muscle cells and intact tissues may be due to the fact that single cells are mechanically free from adjacent cells and connective tissues. It is also possible that membrane properties change during isolation, particularly when cells are isolated in Ca free medium.