Olfactory bulbar electrical responses elicited by nasal infusion of food extracts, hand rinse, and several amino acid solutions were studied in sockeye (Oncorhynchus nerka) and coho (O. kisutch) salmon from several different sources. Comparison was also made in rainbow trout (Salmo gairdneri). Significant variations in the olfactory bulbar responses were observed among different species and even among the same species from different sources. The estimated lowest threshold concentration for L-serine was between 10−6 and 10−7 M in salmon. Olfactory responses of rainbow trout were always several times larger than those of salmon.Pretreatment of nasal cavity with 10−4 M solutions of HgCl2 or CuSO4 (27 or 16 mg/liter, respectively) for 10 sec completely blocked the olfactory responses in all fish species studied. The effect was reversible; the responses returned to their original level by rinsing. Exposure of fish to 0.1 mg/liter HgCl2 for up to 3 days reduced the olfactory responses to 40–10% of those of normal fish. CuSO4 was the more toxic.