Action of pentobarbitone was studied on isolated segments of the terminal and middle ileum of the guinea-pig and compared with the effects of verapamil. Pentobarbitone (10(-4)-10(-3) M) was found to almost equally inhibit responses produced by ES at 3 Hz and ACH, and those elicited by ES at 30 Hz and NA. Increase in the extracellular calcium did not antagonize the effects of pentobarbitone on both types of ES. These results are an indication of a postsynaptic site of action. The concentration-response curves to calcium-induced contractions (3-300 microM) were displaced to the right by both drugs; however, unlike verapamil, pentobarbitone markedly reduced the maximum response to calcium. Calcium-induced contractions after reaching the plateau level were relaxed both by pentobarbitone and verapamil. Pentobarbitone lowered the resting tone even in the high-KCl depolarizing solution, while verapamil produced no change in this series of experiments. It could be concluded that pentobarbitone when used in anesthetic concentrations might affect contractility of the intestinal smooth muscle by an action located at the postsynaptic membrane or beyond it. The antagonism between pentobarbitone and calcium appeared to be noncompetitive and is presumed to be effected via an indirect action on the functioning of calcium channels.