The radial gradient of long-lived corotating energetic particle streams is measured by using observations of 0.9-to 2.2-MeV protons from Helios 1 and 2 between 0.3 and 1 AU, Imp 7 at 1 AU, Pioneer 11 at 3.8 AU, and Pioneer 10 between 9 and 10 AU. These studies cover several time periods from mid-1973 to mid-1976. A positive gradient of ~350%/AU is found between 0.3 and 1 AU. Between 1 AU and some 3--5 AU the gradient is variable with an average value of 100%/AU, which is consistent with the earlier statistical results obtained from the Imp 7 and Pioneer 11 data. A comparison of measurements at 9 and ~4 AU shows a negative gradient which is variable from -40%/AU to-100%AU. Possible solar latitudinal effects on these gradient studies are also discussed. With the use of solar wind and magnetic field data from Helios 1 between 1 and 0.3 AU the relation between corotating energetic particle events in the inner solar system and the interplanetary medium is examined. It is found that the energetic particles are contained inside the high-speed solar wind stream in a region adjacent to the interaction region between low-speed and high-speed streams.