The effect of phytohemagglutinin (PHA) stimulated human lymphocytes on the contractile activity of isolated rat atria was studied. Increased isometric developed tension (IDT) and higher frequency of contractions (FC) were observed shortly after contact of PHA-activated lymphocytes with spontaneously beating rat atria. Since pre-exposure of the lymphocytes to PHA was not necessary and the pharmacologic effects were demonstrated immediately after addition of the lectin, division of the effector cells as a requisite for their action was excluded. Experiments performed with the antibiotic crystallized from Streptomyces chartreusenis, Ca2+-ionophore A-23187, yielded similar effects. Lymphocyte fractionation studies showed that T-lymphocyte-rich and T-lymphocyte-depleted cell fractions had opposite effects on IDT and FC. The inotropic and chronotropic effects were not modified by (-)-propranolol or antihistaminics. Inhibitor of the synthesis of prostaglandins enhanced the action of PHA-activated lymphocytes but inhibitors of the lipoxygenase pathway: 5,8,11,14-eicosatetraynoic acid (ETYA) and nordihydroguaiaretic acid (NDGA) prevented the increment of IDT and FC. Also, FPL55712, an antagonist of SRS-A abolished the positive inotropic and chronotropic effects of PHA-activated lymphocytes on rat atria. Therefore, a central role for SRS-A in this reaction is suggested.