Effects of sodium naphthenates (NAs) on root hy- draulic conductivity (Lp) and gas exchange processes were ex- amined in aspen (Populus tremuloides Michx.) seedlings grown in solution culture. Exposure of roots to NAs for 3 – 5 weeks significantly decreased L p and stomatal conduc- tance. Root-absorbed NAs also decreased leaf chlorophyll concentration, net photosynthesis and leaf growth. Short-term (≤ 2 h) exposure of excised roots to NAs significantly de- creased root water flow (Q v) with a concomitant decline in root respiration. We conclude that NAs metabolically inhibited Lp, likely by affecting water channel activity, and that this inhibi- tion could be responsible for the observed reductions in gas ex- change and leaf growth.