We present a study of the spectral properties of terrestrial kilometric radiation (TKR) derived from observations made by the Goddard radio astronomy experiments on board the Imp 6 and Radio Astronomy Explorer 2 spacecraft. As viewed from near the equatorial plane, TKR is most intense and most often observed in the 2100--2400 LT zone and is rarely seen in the 0900--1200 LT zone. The absolute flux levels in the 100 to 600-kHz TKR band increase significantly with increasing substorm activity as inferred from the auroral electrojet index (AE). In the late evening sector the median power increases by about 3 orders of magnitude between quiet periods (AE200&ggr;). The peak flux density usually occurs near 250 kHz, although the frequency of the peak in the flux spectrum appears to vary inversely with AE from a maximum near 300 kHz during very quiet times to a minimum below 200 kHz during very disturbed times. The half-power bandwidth is typically 100% of the peak frequency. The variation of TKR flux density with apparent source altitude indicates that source strength decreases more rapidly than R-2.