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Effects of sodium naphthenates (NAs) on root hydraulic conductivity (L_p) and gas exchange processes were examined 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 conductance. Root-absorbed NAs also decreased leaf chlorophyll concentration, net photosynthesis and leaf growth. Short-term (≤ 2 h) exposure of excised roots to NAs significantly decreased root water flow (Q_v) with a concomitant decline in root respiration. We conclude that NAs metabolically inhibited L_p, likely by affecting water channel activity, and that this inhibition could be responsible for the observed reductions in gas exchange and leaf growth.