Pressure-probe measurements showed that the pressure relaxation of internodal cells of the freshwater alga Chara connivens slowed considerably when 1–5 mol m−3 Zn2+, or more especially Zn2+ and 75 mol m−3 NaCl, were present in the medium for periods of 1 h or longer. These results indicate that the water permeability of the Chara membrane is decreased by Zn2+, and that this effect is enhanced by 75 mol m−3 NaCl. Specific values taken after 375 min exposure were: 5 mol m−3 Zn2+ and 75 mol m−3 NaCl caused the half-time for bulk water movement to increase from 7·8±2·3 to 79·5±5·4s, corresponding to a decrease in the hydraulic conductivity (Lp) from (13·0±3·3) × 10−7 m s−1 mPa−1 to (1·25±0·23) × 10−7 m s−1 MPa−1 (mean±S.D., n= 10). These changes are not seen in the presence of NaCl alone, and to a reduced extent in the presence of 5 mol m−3Zn2+ alone (after 375 min, Lp was (2·4±0·1) × 10−7 m s−1 MPa−1, mean±S.D., n = 6). Ca2+ cannot substitute for Zn2+, but seems to competitively inhibit Zn2+. There was another, kinetically distinct effect of Zn2+: the ingress of Na+ within 15 min of exposure to 75 mol m−3 NaCl is halved by the presence of 1–5 mol m−3 Zn2+, although internal osmolality is little changed by Zn2+. In spite of this, Zn2+ does not exert the long-term protection against NaCl that has been reported for Ca2+. Depending on the concentration of Zn2+ and the duration of the exposure, the effects on water permeability were fully or partly reversible within 24–48 h. The mechanism of these changes is difficult to identify. One possibility is a zinc-induced restriction of trans-membrane channels to give single-file channels which can be blocked by salt.