The dissociation constants of many phenolic amines, including benzylamines, phenethylamines, phenylethanolamines, phenylpropylamines, catecholamines, and apomorphine have been measured by potentiometric titration at 25 degrees C. Measurements have also been made with many of their methoxy derivatives and with series of phenolic quaternary ammonium salts. Some compounds were also studied at 37 degrees C. 2 Usually at least five titrations were made with each compound and Debye--Hückel theory was applied to convert concentrations to activities but the estimates of pKa were not constant and found to increase with increasing concentration. The range studied was usually 5-15 mM and a least-squares line-fit, based on the empirical assumption that pKa varies with (concentration)1/2, has been used to calculate values for 10 mM solutions and to extrapolate to infinite dilution and to 100 mM. The dependence of pKa on concentration was much less at 37 degrees C than at 25 degrees C. 3 At 37 degrees C the pKa values of many biologically interesting compounds in the group, dopamine, noradrenaline, adrenaline and isoprenaline, coryneine (the trimethylammonium derivative of dopamine) and apomorphine are within 1 log unit of physiological pH, indicating the presence of a significant proportion of either the zwitterion or of the uncharged phenolic amine. 4 Zwitterion constants have been estimated from the pKa values of the phenolic amines and those of their methoxy and quaternary trimethylammonium analogues. Zwitterion formation does not appear to be associated with activity at alpha-adrenoceptors and probably not with activity at beta-receptors. The active species seems likely to contain the unionised phenolic group but at dopamine receptors this may be in the uncharged phenolic amine rather than in the phenolic ammonium salt.