Abstract: A medical probe having at least one transducer or electrode for registering or influencing tissue activities is disclosed. The probe comprises a substrate having a supporting surface and a cover layer which, together, form a chamber bounded on one side by the supporting surface and on an opposed side by a wall of the cover layer. The electrode or transducer is carried on the supporting surface of the substrate. An aperture in the wall of the cover layer provides communication between the exterior of the probe and the chamber. The probe is extremely accurate and selective, and extraneous signals which made prior art probes inaccurate do not detract from the accuracy of probes according to the instant invention.