A prolonged contraction at the end-plate region of the diaphragm of rats and mice after anticholinesterases in vitro.

1. Experiments have been made on the phrenic nerve and diaphragm of rats and mice in vitro at 37 degrees C. 2. The administration of the anticholinesterase drugs ecothiopate, ambenonium or neostigmine resulted in enhancement of the 'twitch' response to nerve stimulation which was often followed by a prolonged contraction lasting several seconds. 3. The prolonged contraction was localized at the end-plate. 4. Associated with the prolonged localized contractions were prolonged end-plate currents and prolonged end-plate potentials. 5. Procedures which diminished transmitter release or action, i.e. high [Mg2+]o, alpha-bungarotoxin, (+)-tubocurarine, denervation and repetitive stimulation, also diminished or curtailed the prolonged localized contraction. 6. It is concluded that the prolonged localized contractions are due to prolonged transmitter actions as a consequence of inhibition of the acetylcholinesterase, at the end-plate.