Taurine, a principal osmolyte in molluscan integument, is actively transported from sea water by Mytilus gill by means of a Na(+)-dependent process. In this study we examined the response of this transport to reductions in external salinity, i.e. the response to reductions in osmotic concentration as well as Na+ concentration. Acute exposure of isolated gill tissue to 60% artificial sea water (ASW) resulted in a greater than 85% inhibition of taurine uptake, substantially more than the 45% inhibition predicted on the basis of the acute reduction in external [Na+]. Within 60 min, however, taurine transport recovered to the level predicted by the Na+ concentration in dilute sea water. Isolated gills acutely exposed to 60% ASW made isosmotic to normal (100%) ASW with mannitol had rates of taurine uptake comparable to gills acclimated for 60 min. Taurine uptake by gill tissue exposed to 60% ASW for 60 min and then returned to 100% ASW for 90 min was not significantly different from that of control gills held in 100% ASW. Glucose uptake by the gill during acute exposure to reduced salinity responded in a pattern similar to that of taurine. Gill tissue increased by 20% in wet mass within 2 min of exposure to 60% ASW, but returned to control mass within 30-60 min, presumably reflecting cell volume regulation. Long-term (12 days) exposure to reduced salinities was not accompanied by increases in taurine transport over that of gills observed following the 60 min 'short-term' acclimation. These results suggest that Mytilus gill undergoes a rapid (albeit incomplete) recovery from the extreme inhibition of transport associated with abrupt changes in salinity, and the extent of recovery is defined by the availability of Na+ in the external medium. The extreme sensitivity of taurine uptake observed after acute exposure of gills to reduced salinity is related to the osmotic concentration of the medium, and is possibly linked to a change in cell volume.