The catalytic and inhibitory profiles of xylose reductase isolated from the yeast Pachysolen tannophilus (PTXR) are compared to those of aldose reductase (AR) obtained from rat lens. While both PTXR and rat lens AR are NADPH-specific enzymes and have an affinity for a variety of substrates such as D-xylose, D,L-glyceraldehyde, and 4-nitrobenzaldehyde, the enzymes differ in their substrate affinity profiles. Also, PTXR is not inhibited by standard inhibitors of AR thus supporting a hypothesis that this enzyme may not possess the inhibitor binding site found in rat lens AR.