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The commercially available transducers for direct measurement of myocardial tension are either large in size or lack the ability of isometric measurement. Moreover, none of them enables measurement of local isometric tension just around the site of electrical stimulation of the heart. A miniature strain gauge transducer was developed in our laboratories and is herewith described. It weighs 150 mg, and consists of two arms with adjustable distance between 1-5 mm. It is applied to the heart by 2 needles which may serve also as stimulating or sensing electrodes. A full bridge made of constantan foil strain gauges ensures low drift. The device enables in vivo measurement of isometric tension of a myocardial segment of the site where the tension is being measured and permits simultaneous recording of the electrogram. Attachment of the gauge to the ventricular wall by means of needles makes simple the transfer of the transducer from one measuring point to another. The performance of the miniature force transducer was compared to that of a standard Walton-Brodie strain gauge arch demonstrating marked similarity of response at low heart rates. At high frequencies however differences in mechanical response of the heart were obtained by the two force transducers.