Fifty-two adult male sharp-tailed grouse were live-trapped on the breeding grounds in Montana and administered single, oral doses of dieldrin, malathion, or lactose, and released. Reactions to the insecticides were determined by observing neck-tagged grouse on the breeding grounds and by radio-tracking grouse carrying transmitters. Lethal doses of dieldrin ranged from 5.0 to 32.2 mg/kg and the calculated LD50 was 6.9 mg/kg. Results with malathion-dosed birds were more variable, but the lethal dosage appeared to lie between 200 and 240 mg/kg. Fourteen procedural control birds all survived and continued their breeding activity normally. These results generally were comparable to results of tests with penned sharptails. In the wild grouse, however, there was some evidence that birds dosed at sublethal levels were more than normally vulnerable to predators. Some interesting behavioral responses were also observed. Changes occurred in social hierarchy and effective breeding behavior was terminated in some of the grouse treated at sublethal levels.