A new system developed for calibrating the horizontal incidence pyrheliometer is described. The pyrheliometers to be calibrated are exposed simultaneously with a standard pyrheliometer in an integrating sphere. Calibrations are made by comparing voltages developed by the instruments undergoing calibration with those of the standard pyrheliometer. Calibration of the standard pyrheliometer is based on comparisons with the Smithsonian Institution pyranometer, both out-of-doors on clear days and within the integrating sphere. Advantages of the new system include reproducibility of the calibration within less than one percent. This is due to the reproducibility of the radiation field in the integrating sphere, in which there are relatively small variations in ambient temperature. The calibrations can be done much more rapidly and accurately than was formerly the case when the work was done out-of-doors; clear skies and minimum atmospheric pollution were necessary conditions previously.