1 The dissociation constants of 5-hydroxytryptamine and 5-hydroxy-N,N-dimethyltryptamine have been measured and the zwitterion constants have been estimated from the pKas of analogous methoxyamines and the phenolic quaternary ammonium salt. 2 Direct measurement of zwitterion constants has been made by a spectroscopic method which has been used also with several phenolic amines previously studied electrometrically. It make fewer assumptions and so should be more reliable but an appraisal of the methods available indicates that none can be singled out as being best and it is desirable to obtain results with as many as possible. 3 The results consolidate previous observations on the relations between chemical structure and zwitterion constant; possibly zwitterions are stabilized by any factor which stabilizes water structure. Dimethylamino compounds have lower zwitterion constants than their methylamino or aminoanalogues but there is no reason to doubt the previous finding that diethylamino compounds have higher zwitterion constants than their dimethylamino analogues. 4 The proportion of 5-hydroxytryptamine present as the neutral molecule or zwitterion in physiological conditions is small; however, with dopamine, the proportion present as the zwitterion could be as much as 10%.