We employed the whole-cell patch clamp technique to investigate the effects of arachidonic acid (AA) on barium inward current through the L-type calcium channels (IBa) and on osmotic stretch-induced increase of IBa in guinea-pig antral gastric myocytes. Under isosmotic condition, AA inhibited IBa in a dose-dependent manner to 91.1⁑1.4, 72.0⁑3.2, 46.0⁑1.8, and 20.3⁑2.3% at 1, 5, 10, 30 mM, respectively. The inhibitory effect of AA was not affected by 10 ㄍM indomethacin, a cyclooxygenase inhibitor. Other unsaturated fatty acids, linoleic acid (LA) and oleic acid (OA) were also found to suppress IBa but stearic acid (SA), a saturated fatty acid, had no inhibitory effect on IBa. The potency sequence of these inhibitory effects was AA (79.7⁑2.3%) ＞ LA (43.1⁑2.7%) ＞ OA (14.2⁑1.1%) at 30 ㄍM. On superfusing the myocyte with hyposmotic solution (214 mOsm) the amplitude of IBa at 0 mV increased (38.0⁑5.5%); this increase was completely blocked by pretreatment with 30 mM AA, but not significantly inhibited by lower concentrations of AA (1, 5 and 10 ㄍM) (P＞0.05). Unsaturated fatty acids shifted the steady-state inactivation curves of IBa to the left; the extent of shift caused by AA was greater than that caused by LA. The activation curve was not affected by AA or LA. The results suggest that AA and other unsaturated fatty acids directly modulate L-type calcium channels and AA might modulate the hyposmotic stretch- induced increase of L-type calcium channel current in guinea-pig gastric smooth muscle.