In the rabbit iris sphincter muscle, electrical transmural stimulation produced fast and slow components of contraction which were markedly attenuated by tetrodotoxin. The fast component was augmented by physostigmine and was abolished by atropine, while the slow component was little affected. Adrenergic and ganglionic blocking agents did not inhibit the slow component. Therefore, the fast component is probably cholinergic, while the slow one is noncholinergic, nonadrenergic in nature. Capsaicin did produce a considerable contractile response, but there was a gradual decline with repetitive application and a tachyphylaxis occurred. Under such conditions the slow but not the fast component was abolished. Substance P and acetylcholine produced the largest contraction, while ATP, histamine, serotonin and noradrenaline produced little or no response. In cold stored preparations, the responses to electrical transmural stimulation and capsaicin were either markedly attenuated or abolished, whereas substance P and acetylcholine produced considerable contractions. Baclofen and theophylline did not inhibit the slow response to electrical transmural stimulation or the response to substance P and capsaicin. Thus, electrical transmural stimulation produces cholinergic and noncholinergic, non-adrenergic contractions in the rabbit iris sphincter muscle and the latter response is considered to be mediated by substance P or a related peptide released from the neural component.