Using a 5-odorant identification confusion matrix task, the authors assessed the consequence of olfactory epithelial damage on odorant quality perception in the rat. After establishing prelesion identification performance, each rat's epithelium was subjected to 330 ppm methyl bromide gas for 6 hr. Comparison of prelesion and 3-day postlesion performance demonstrated a significant decrease in identification as a consequence of 95%-98% epithelial destruction. Further, there was a differential effect of lesion on the ability of different animals to identify the different individual odorants. Evaluation of the anatomical state of the epithelium relative to performance on the identification task demonstrated a significant relationship between the extent and location of anatomical sparing and changes in individual odorant identifications. Assessment of pre- and postlesion quality perception for the individual rats demonstrated a highly significant shift in quality perception that was independent of any decrease in performance. These results provide strong support for the proposition that the regional variations in mucosal sensitivities within and across olfactory receptor gene expression zones are fundamentally important for the encoding of odorant quality.