Normalized difference vegetation index (NDVI) data obtained from the Advanced Very High Resolution Radiometer (AVHRR) on board NOAA-9 have been analysed to assess their utility for monitoring the vegetation of Tunisian grazing lands. Preliminary analysis shows that the NDVI provides a sensitive indicator of monthly variations in biomass which correlate with spatial and temporal changes in growing conditions. Investigations suggest that the percentage contribution of the soil background to total recorded reflectance, provides an important limiting factor to the sensitivity of the NDVI, creating a threshold beyond which the accuracy of this index becomes less reliable.