The spectral density of channel current noise in JFET's is governed in the low-frequency region by Lorentzian terms. Ordinarily, their influence on the equivalent noise charge of preamplifiers for radiation detectors is negligible, as these terms vanish as soon as the frequency exceeds a few Hz. Exposure of JFET's to γ - rays has proved, however, that these noise contributions are enhanced by the absorbed radiation and may invade a much broader frequency range, thereby degrading the equivalent noise charge event at comparatively short processing times. The extent to which the noise behaviour of JFET's is affected by radiation-enhanced Lorentzian terms is analysed in this paper. Results of measurements, carried out after exposure to 60CO γ- rays of JFET parts and monolithic circuits realised in two different technologies, are discussed and compared with the theoretical predictions.