In testing poorly soluble substances in-vitro on isolated organs, organic solvents and solubilizers are used to increase water-solubility. To facilitate selection of appropriate substances, the effects of eleven of these chemicals have been studied in the following isolated smooth muscle preparations: guinea-pig ileum stimulated by carbachol, histamine, 5-HT or single field stimuli; rat fundus stimulated by 5-HT; and mouse vas deferens stimulated by noradrenaline or trains of field stimuli. Nine solvents (acetone, diethyleneglycol monoethylether, dimethyl sulphoxide, ethanol, glycerol, methanol, polyethylene glycol 400, 1,2-propanediol, Tetraglycol (tetrahydrofurfurylalcohol polyethyleneglycolether)) and two detergents (Triton-X 100 and Tween 80) were examined. The vas deferens proved to be most resistant, whereas rat fundus and guinea-pig ileum were more sensitive to the effects of solvent when present from 1 to 10 g L-1. Most solvents caused non-specific, concentration-dependent reversible inhibition of contractions. Dimethyl sulphoxide in high concentrations increased the contractile responses of guinea-pig ileum stimulated by 5-HT and in both experiments with electrical stimulation. Polyethylene glycol 400 augmented the response of mouse vas deferens to electrical stimulation. Overall, 1,2-propylene glycol and polyethylene glycol 400 had the least effect and can be used in a concentration of 3 g L-1, and in qualitative studies even up to 10 g L-1. Glycerol, both monohydric alcohols and dimethyl sulphoxide produced more intense effects and should not exceed concentrations of 1-3 g L-1. Stronger inhibition was caused by diethyleneglycol monoethylether, acetone and Tetraglycol, and the bath concentrations of these substances should not exceed 0.5-1g L-1. Of the detergents only Tween 80 is suitable as a solubilizer in smooth muscle preparations in-vitro, forming micelles at 10 mg L-1 a concentration tolerated by isolated organs in this study.