The effects of lithium carbonate on the responses to five neuromuscular blocking agents were evaluated in dogs anesthetized with halothane (1 per cent) and N2O (60 per cent) in O2. Latency (time from first twitch-height depression to maximal blockade), maximal twitch-height depression, and times to return to 50 per cent and 100 per cent control twitch tension were measured before and after intravenous infusion of lithium carbonate (1 mg/kg/min for one hour) during neuromuscular blockades produced by succinylcholine, decamethonium, gallamine, d-tubocurarine, or pancuronium. Lithium prolonged the latencies of neuromuscular blockades produced by 0.1 mg/kg succinylcholine and 0.1 mg/kg decamethonium by 248.1 per cent and 49.0 per cent, respectively, but had no effect on latency produced by 0.02 mg/kg pancuronium. The times for return to 50 per cent of control twitch height were prolonged by 69.5, 40.0, and 120.1 per cent, respectively. Lithium had no effect on latency or duration of blockades produced by 0.15 mg/kg d-tubocurarine and 0.6 mg/kg gallamine, but enhanced maximal twitch-height depressions produced by 0.9 mg/kg gallamine and 0.02 mg/kg pancuronium by 22.9 and 9.9 per cent, respectively. Twitch tensions decreased 5-10 per cent over three hours in three dogs receiving lithium infusion without relaxants. Twitch tension was depressed 0-2 per cent in three dogs after five hours of anesthesia in the absence of lithium or relaxants. Lithium prolonged the time required for neostigmine to reverse neuromuscular blockade produced by pancuronium in two of three dogs from a mean of 60 seconds to 135 seconds.