Electrodermal activity was measured in a group of depressive patients and normal controls. Those patients classified as endogenous on the Newcastle Scale had significantly lower skin conductance levels than either the non-endogenous patients or controls. The endogenous depressives also contained significantly more non-responders to the experimental stimuli. The lack of responsiveness to external stimulation in non-responders is associated with a significantly lower rate of blood platelet 5-hydroxytryptamine uptake, suggesting that endogenous depressives have biological characteristics that distinguish them from other depressive groups and that electrodermal measures may be useful in the classification of depressive illness.