Effect of gamma-aminobutyric acid (GABA) and its agonists, phenibut, phenylpyrrolidone and LGPI-29, on contractility of smooth muscle cells was studied on an isolated rat vena porta. The contractility was recorded by a rapidly operating 6MH-1B mechanotrone to show that the solutions of GABA and those of phenibut (to a less extent) exert a distinct inhibitory effect on smooth muscle cells of the rat vena porta at a concentration of 10--20 mM. Phenylpyrrolidone and LGPI-29 did not decrease vascular tone. Depolarization of the cell membrane did not interfere with the inhibitory effect of GABA and its agonists on smooth muscle cell contractility of the rat vena porta.