We studied the effect of moderate heat stress (30oC) and muscarinic cholinergic receptor agonists arecoline and pilocarpine on sensitivity of the behavior of the nematode Caenorhabditis elegans of N2 line to the action of the agonist of nicotinic cholinergic receptor agonist levamisole. Heat stress and muscarinic cholinergic receptor agonists increased the sensitivity of swimming induced by mechanical stimulation to levamisole (32-64 μM), which manifested in dyscoordination of locomotor muscles during swimming and complete loss of ability to swim. Combined exposure to heat stress and muscarinic cholinergic receptor agonists revealed their synergism in the influence on sensitivity of swimming behavior to levamisole: heating to 30oC potentiated the effect of arecoline and arecoline potentiated the effect of heat stress. It is assumed, that the effect of heat stress on the sensitivity of nicotinic receptors is mediated by its effect on muscarinic receptors.