Time series of reflectances over the Valensole plateau test site (southeast of France) obtained from NOAA-9 Advanced Very High Resolution Radiometer (AVHRR) data show significant short-term variations over a three-month period. This paper demonstrates that the short-term variations are due essentially to surface bidirectional effects while atmospheric directional effects and other sources of fluctuations remain of lower amplitude. When the data are classified according to their viewing angles, in order to reduce the influence of directional effects, the consistency of the NDVI time series is strongly improved and its r.m.s. level of short-term fluctuations is decreased by a factor of 2, from 005 to 0025. Taking into account the effects induced by viewing angle variations should allow us to improve definitely the accuracy of the characterization and monitoring of the terrestrial vegetation.