1. The Na+, K+-activated, Mg2+-dependent-ATPase enzyme system in a heavy microsomal fraction of rainbow trout (Salmo gairdneri) brain was inhibited in vitro by chlorinated hydrocarbon pesticides.
2. T50 (concentration at 50 per cent inhibition) values for dicofol, endosulfan and DDT were 5 × 10−6, 3 × 10−5 and 1 × 10−4 M respectively. Similar inhibition by these pesticides occurred in kidney and gill ATPase preparations.
3. An unexpected finding was a failure of the classic inhibitor, ouabain, to block the Na+, K+-activated component of ATPase activity in the gill.
4. It is suggested that inhibition of ATPase activity may be a causal factor in the toxic effects of organochlorine pesticides in fishes.