An instrument to detect the magnetic components of ELF signals propagating in the magnetosphere was carried on Explorer 45. Over 600 hours of observations in the inner magnetosphere near the equatorial plane have been examined. These observations were obtained in late 1971 and the first half of 1972 when the satellite apogee was in the evening and afternoon quadrants. The strongest most persistent signals were plasmaspheric hiss from a few hundred to a few thousand hertz. Broad band signals of 25 m&ggr; were common. Frequently, the hiss terminated abruptly during a satellite pass near and inside the boundary of the plasmasphere. Hiss boundaries were observed usually beyond L=4 in quiet times, Kp=0 to 1+, but were frequently beyond apogee near L=5. During disturbed times, Kp>4+, hiss boundaries remained near L=5 from 1600 to 1900 LT but were below L=4 from 2000 to 2400 LT. The magnetic index best correlated with the hiss boundary near midnight was Dst, the ring current index. The boundary location near midnight ranged from L=2.5 for Dst=-160 &ggr; to L=5.5 for Dst=0. The peak intensity of hiss during an orbit occurred most frequently on the afternoon about 1 Re inside the hiss boundary. The most intense hiss was observed in the recovery phase of magnetic storms at the inner edge of the ring current. The source of the hiss appears to be the outer plasmasphere. Generation of hiss through cyclotron resonance with energetic electrons is the probable source for most of the hiss. Ring current protons, forming a peak in the proton flux between 10 and 100 keV, may be a source for some of the hiss.