The responses of muscle effectors of penile erection to drugs and to electrical stimulation in various animals were compared. Tissues used were the retractor penis (RP) of rat, dog, cat, horse, boar, hog, elk, bull, ram and goat; corpora cavernosa urethrae of macaque, rabbit, guinea pig, dog, cat and horse; and penile artery of the bull. Preparations were suspended in Tyrode for drug treatment or mounted between platinum electrodes for field stimulation. All except boar RP showed spontaneous activity, unaffected by tetrodotoxin (TTX, Sankyo) and often triggered by noradrenaline (NA, Sigma-Chem.). 2/3 Were not contracted by ACh (Roche), and some did not contract even in the presence of physostigmine (Sandoz); ACh response was abolished by atropine (ACO) or scopolamine (Sigma-Chem.). Nicotine (BDH) relaxed all except boar RP, most requiring pretreatment with 6OH-dopamine (Fluke), this effect was blocked by hexamethonium (May+Baker), mecamylamine (MSD) or lidocaine (Astra), but not by TTX, All were contracted by adrenaline, NA, and dopamine (Sigma-Chem.), this effect being abolished by phentolamine or phenoxybenzamine (SK+F). Isoprenaline (Abbott) relaxed most, and this action was blocked by propranolol (ICI). 5-OH-tryptamine (Fluke) induced variable responses, the contractile response being blocked by methysergide (Sandoz). The response to histamine (Sigma-Chem.) was also variable, an excitatory effect in bull RP being increased by dipyridamole. MCGE1, E2 and F2a (-stra), substance P (Beckman), bradykinin (Sandoz), isoleucine5iotensin (Beckman), valine5-angiotensin (CIBA-Geigy), oxytocin and arginine- and lysine-vasopressin (Ferring) all produced species-variable responses. Excitatory responses to field stimulation were blocked by guanethidine (CIBA -Geigy), bretylium (Wellcome), or 6-OH-dopamine. LSD (Sandoz) contracted most. The inhibitory response to field stimulation, induced after pretreatment with ascorbic acid + guanethidine or 6-OH-dopamine, or sometimes NA, was abolished by TTX or lidocaine, but was not blocked by a number of drugs already mentioned, or by d-tubocurarine, metoclopramide (Lunbeck), cyproheptadine (MSD), mepyramine (Pharma-Rhodia), metiamide or cimetidine (both SK+F), chlorpromazine (Medica), thioridazine (Sandoz), indomethacin (MSD), lithium, ouabain (Sandoz), SKF-525A (proadifen (SK+F) or colchicine (Star).