Dopa decarboxylase (DDC) is involved directly in the synthesis of dopamine and serotonin and indirectly in the synthesis of noradrenaline. The present study investigated two Danish families for linkage between manic depressive illness, a marker at the DDC locus which has been mapped to 7p11-p13 and 10 microsatellite markers covering chromosome 7q11-p15. No evidence of linkage was found assuming a dominant or a recessive mode of inheritance. We have earlier reported evidence against linkage between manic depressive illness and tyrosine hydroxylase and dopamine beta-hydroxylase. Mutations of major importance in the genes encoding the three enzymes involved in the synthesis of dopamine and noradrenaline seem less likely in the families we have studied. Further investigations of genetic and other pathophysiological mechanisms in relation to the monoamine hypotheses for manic depressive illness are still of relevance. There is a need for further linkage and association studies as well as a search for possible mutations in the relevant genes involved in the monoaminergic pathways to clarify their possible role in the aetiology of manic depressive illness.