We report herein the synthesis and antiarrhythmic activities of some newly synthesized heterocyclic theino[2,3-c]pyrimidine and theino[2,3-c]pyridine derivatives fused with thiophene moiety. Initially the acute toxicity of the compounds was assayed via the determination of their LD(50). The antiarrhythmic activities for the compounds were determined and all the tested compounds were found more potent than Procaine amide (R) and Lidocaine (R) as positive antiarrhythmic controls.