Field gamma-ray spectrometry is a rapid and effective quantitative method of mapping variations of radioelements within igneous intrusions. A field procedure for radioelement mapping demonstrates the value of the method to studies of late-stage magmatic processes during the emplacement of granite intrusions and the determination of their present-day heat productivities. Methods and problems of instrument calibration using both natural and artificial sources are discussed. Calibration based on neutron activation analysis of samples from natural outcrops achieves results comparable with those obtained using artificial sources and has the advantage that it relates directly to field conditions; furthermore it enables secular disequilibrium in the uranium and thorium decay series to be recognized.