The cardiac actions of arachidonic acid (AA, C 20:4), the precursor of prostaglandin E2 (PGE2) and F2 alpha (PGF2 alpha) were studied in isolated atria and papillary muscles of guniea-pigs as well as canine Purkinje fibers. In contrast to the cardio-depressant actions of some short and long-chain fatty acids (octanoate, oleate) AA (10(-6)-10(-4) M) increased sinoatrial rate and contractility in guinea-pig atrial preparations. This cardio-stimulant action of AA was accompanied by an increase in the amplitude and maximum rate of rise of the intracellularly recorded atrial action potential. Some increase in the resting membrane potential and a slight prolongation of the action potential duration could also be observed. AA did not induce any pace-maker activity in isolated papillary muscle of guinea pigs or in isolated Purkinje fibers. Similarly to AA, PGE2 and PGF2 alpha (2.1-10(-8)12.1-10(-5) M) were also found to increase atrial rate and contractile force. The stimulating actions of AA were reduced by application of high concentrations of indomethacin (5-10(-5) M).