Results from the first 3 years of a continuing program to monitor time variations in the intensity and polarization of the 13-cm synchrotron emission from Jupiter's radiation belts are reported. After 3 or 4 years of reduced emission at decimetric wavelengths the 13-cm flux density began to increase during 1973, and the increase accelerated throughout 1974. At the time of the Pionner 11 encounter the synchrotron emission was ~15% greater than it was during the Pioneer 10 encounter. When the new data are combined with previousl published 11- and 21-cm measurements, it is apparent that the nature of the time variations during the past 13 years is similar but not identical at these wavelengths and that there is no convincing evidence that the variations are correlated with either solar activity or Jupiter's distance from the sun. During the same time interval, measurements of the peak degree of linear polariation of the synchrotron emission, with one exception, remained constant near 24.5%.