The effects of the newly synthesized neuromuscular blocking agent, chandonium iodide (17a-methyl-3β-pyrrolidino-17a-aza-D-homo-5-androstene dimethiodide) have been investigated on guinea-pig isolated ileum and vas deferens preparations. On the ileum, chandonium (0·1–10·0 μg ml−1; 1·6 × 10−7m–1·;6 × 10−5m) had weak muscarinic receptor blocking action (pA2 is 5·7), but no antihistamine properties at the concentration tested. No evidence for anticholinesterase actions was found. On the vas deferens, chandonium (10·50 μg ml−1; 1·6–8·1 × 10−6 m) potentiated responses to exogenous noradrenaline; responses to electrical stimulation were potentiated only in the presence of 50 μg ml−1 chandonium. No adrenoceptor or adrenergic neuron blockade was found. The results provide evidence that chandonium acts selectively at acetylcholine receptors and that it is more active as a nicotinic receptor antagonist than as a muscarinic receptor antagonist.