The long term effects of three sublethal concentrations of zinc (0.214, 0.52 and 1.12 ppm) on growth and plasma glucose concentration in yearling rainbow trout Salmo gairdneri (Richardson) were investigated. Analysis of covariance of percent weight increase revealed that a significant inhibition of growth (P<0.05) in the 1.12 ppm zinc-exposed fish had occurred. Plasma glucose showed a significant hyperglycemia (P<0.05) in all three zinc-exposed groups of fish after 7 days exposure and in the 1.12 ppm zinc-exposed group after 63 days. The hyperglycemia observed has been explained as possibly resulting from activation of the pituitary-interrenal axis by the stress of zinc causing mobilization of tissue glycogen.