The agent, 6-(N, N-diethylamino)-hexyl-3,4,5-trimethoxybenzoate (TMB-6) which relaxes smooth and skeletal muscle by interfering with intracellular Ca++ availability was tested for its electrophysiological effects on canine cardiac Purkinje fibres and feline papillary muscles. In both tissues the drug causes a decrease in resting membrane potential and action potential amplitude as well as spontaneous rate in Purkinje fibres, and an increase in stimulus needed to reach threshold. The minimum effective dose for both tissues was similar at 7.32 × 10–5M. The drug effects were reversible in both tissues upon removal of the TMB-6. The Ca++-deρendent action potentials of canine cardiac Purkinje fibres were also inhibited by TMB-6 at dose range of 7.32 x 10–5 –24.4 x 10–5M. Because of its electrophysiological effects on threshold, automaticity and ‘slow response’ action potentials and its reversibility, TMB-6 could become a useful antiar