Chinook salmonOncorhynchus tshawytschawere injected with d-Ala6, Pro9NEt mammalian gonadotropin releasing hormone (GnRHa) at 5 μg/kg body weight, a typical dose administered to promote ovulation or spermiation. Following this injection, salmon were sampled serially, and plasma GnRHa levels were determined by chromatography and radioimmunoassay. Plasma levels of GnRHa were elevated following injection (>500 pg/mL), then levels declined rapidly and were nondetectable in all fish by 5 h postinjection. Using nonlinear regression, the data were fitted to an exponential decay model, which calculated a plasma half-life of 0.8 h, with a 95% confidence interval of 0.5-5.8 h. These studies provide further evidence of the rapid clearance of GnRHa, when used for the acceleration of final sexual maturation.