An in vivo NMR probe is disposed at the distal end of a catheter or endoscope for obtaining NMR spectra from within a patient. The probe is constructed from a passive integrated circuit including a receiving coil and a parametric up-converter for increasing the receivied frequency of the NMR emissions. One or more coaxial cables disposed in a lumen of the catheter connect the integrated circuit probe to an external NMR processor. The external processor may also excite the coil to radiate a localized perturbation field prior to obtaining NMR emission data of an area of interest. Alternatively, the probe may be constructed from an active circuit which enables the coil to be tuned to an NMR emission frequency of interest. A closed loop refrigeration circuit utilizing lumens in the catheter or Peltier junction devices on the integrated circuit provide for temperature stability of the active devices.