Radiances obtained from the NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) have been compared with those derived from a coaligned aircraft-mounted double-pass Fastie-Ebert spectrometer when both instruments were simultaneously observing the dunes region of White Sands, New Mexico. The radiance calibration of the airborne spectrometer was arranged to be traceable to NBS standards through the procedures that were used for the prelaunch calibration of the AVHRR. Data from August 1985 and October/November 1986 have been analyzed, and suitable corrections made for the effects of the atmosphere between the U-2 aircraft (flying at a pressure level near 6 N/m2 (60 mb)) and the satellite. For this purpose, atmospheric altitude profiles of temperature, pressure and water vapor were measured from a radiosonde, the most probable ozone profile was derived from statistically inter-preted Nimbus-7/SBUV measurements, and likely stratospheric aerosol conditions were assumed to calculate the ratio (as a function of wavelength) of the radiances expected at each instrument. The calculated ratio was used to adjust the measured U-2 spectra to their equivalent at the altitude of the NOAA-9 AVHRR. The calculations indicated an absolute accuracy of +5% in the derived sensitivity of the AVHRR visible channels relative to the sensitivity measured prelaunch, and that some improvement in absolute accuracy is possible with modest additional effort. The analysis showed that in August, 1985, the sensitivities of channel 1 (570-700 nm) and channel 2 (710-1000nm) of the NOAA-9 AVHRR were indistinguishable from their prelaunch values. In October/November, 1986, sensitivity losses were calculated to be approximately 12% for channel 1, and 19% for channel 2. Flights on three days during the period 24 October to 5 November 1986 reproduced these results to within +1.5% in sensitivity loss. Results for the GOES-6 VISSR visible channel showed a 9-14% loss in sensitivity after 42 months in orbit. For LANDSAT-5 TM a 12% loss in band 4 after 17 months in orbit was indicated, with no discernable change in the sensitivities of bands 1-3.