The effect of ageing and suicide on mu opioid receptors was studied in the human brain postmortem. Quantitative autoradiography with [3H]DAGO revealed region specific increases in mu receptor density with age. Suicide was accompanied by a significant increase, up to 9-fold, in mu receptor density in the young, but not the old, subjects as compared to age-matched controls. This effect was evident in the frontal and temporal cortical gyri. Saturation binding studies with the same ligand show that the increased binding in the elderly and in the young suicides is due to an increase in receptor density (Bmax) rather than affinity (Kd).