A set of miniature isotropic probes has been used experimentally to map electric fields within two small spheres of simulated muscle tissue (3.3-cm and 8-cm radii) that were irradiated by 450-, 915-, and 2450-MHz plane waves. Good agreement was obtained with the theoretically predicted spatial distribution of fields in both spheres, even in the immediate proximity of the boundaries. The absolute calibrations of the probes were found to be moderately to significantly dependent upon the dielectric constant of the medium (1 ≤ ∈r ≤ 50) in which the probe was implanted, with the least dependence occurring at 2450 MHz (±2.25 dB), and the greatest dependence occurring at 450 MHz (±4.25 dB). A set of design criteria is presented that are necessary for proper performance of such a dipole/diode probe. Of particular importance are the thickness of the insulation surrounding the dipole antenna, and the dimensions of the dipole. Both must be small compared to the wavelength of the field in the biological media. The validity of the design criteria has been experimentally demonstrated, illustrating the feasibility of producing an improved probe whose response to an electric field of given strength would be relatively constant regardless of the dielectric constant of the media and independent of boundary-proximity effects. Preliminary results of in vivo measurements are presented; improved, biologically compatible designs are discussed.