ABSTRACT—Thrombocyte monoamine oxidase (MAO) activity and concentrations of 5‐hydroxyindole acetic acid (5‐HIAA) and homovanillic acid (HVA) in the cerebrospinal fluid (CSF) were studied in patients with motor neuron disease (MND) and healthy controls. The MAO activity in MND (n = 32) did not differ from age and sex‐matched controls (n = 27) or from healthy relatives (n = 9) and no difference was found between advanced and mild cases. In an attemtp to explain divergent results from other studies, the effect of storage time on analysis of platelet concentration was studied in three healthy subjects and in three patients with MND. No difference was observed between the groups. No differences were found between the MND group (n = 28) and healthy controls (n = 18) with regard to CSF levels of 5‐HIAA and HVA, except for an increase of 5‐HIAA among males with MND. The results of this study do not confirm earlier studies that have reported extensive changes in the variables analyzed in patien