A new flexible sensor for in vitro experiments was developed to measure the surface potential, Phi, and its gradient, E (electric near field), at given sites of the heart. During depolarisation, E describes a vector loop from which direction and magnitude of local conduction velocity theta can be computed. Four recording silver electrodes (14 microm x 14 microm) separated by 50 microm, conducting leads, and solderable pads were patterned on a 50 microm thick polyimide film. The conductive structures, except the electrodes, were isolated with polyimide, and electrodes were chlorided. Spacer pillars mounted on the tip fulfil two functions: they keep the electrodes 70 microm from the tissue allowing non-contact recording of Phi and prevent lateral slipping. The low mass (9.1 mg) and flexibility (6.33 N/m) of the sensor let it easily follow the movement of the beating heart without notable displacement. We examined the electrodes on criteria like rms-noise of Phi, signal-to-noise ratio of Phi and E, maximum peak-slope recording dPhi/dt, and deviation of local activation time (LAT) from a common signal and obtained values of 24-28 microV, 46 and 41 dB, 497-561 V/s and no differences, respectively. With appropriate data acquisition (sampling rate 100 kHz, 24-bit), we were able to record Phi and to monitor E and theta on-line from beat-to-beat even at heart rates of 600 beats/min. Moreover, this technique can discriminate between uncoupled cardiac activations (as occur in fibrotic tissue) separated by less than 1 mm and 1 ms.