Rainbow trout transferred from isotonic seawater to freshwater showed a direct correlation between nitrite uptake and lamellar chloride cell number over an acclimation period lasting 96 h. After a 48 h acclimation, fish exposed to 175 mg/l of nitrite for 2 1/2 h had significantly higher plasma nitrite levels than fish acclimated for 24 h. There was a 9.5 fold increase in lamellar chloride cell number over this period. The possible role of lamellar chloride cells in the ionic uptake of nitrite is discussed in light of previous research concerning the protective effect of environmental chloride on nitrite toxicity.