A study of psychotropic drug prescribing, derived from the computerized pricing data in Northern Ireland from 1966, showed that the use of these drugs reached a peak in 1975, when about 12·5% of the adult population were estimated to have been receiving them, and declined in the following 5 years. Benzodioazepines accounted for three-quarters of all psychotropic drugs prescribed in 1980. Benzodiazepine tranquillizer prescribing was consistently 20–30% higher than in the rest of the United Kindom, in contrast to hypnotic and antidepressant prescribing which has been consistently lower. The rate of increase in benzodiazepine tranquillizer prescribing was greater than in other European countries, but the level remains lower than in Iceland and Denmark. The influence of a number of demographic and socioeconomic variables was studied in an intra-regional analysis of the 1978 data for the 17 health districts in the province, using multivariate and multiple regression statistics. The prescribing of benzodiazepine hypnotics was almost entirely accounted for by the proportion of elderly (over 65 years) and women aged 45–59 years; neuroleptic prescribing was largely a function of factors associated with rural areas (overcrowding and unemployment) and the proportion of elderly; but neither tranquillizer, antidepressant, barbiturate hypnotic nor psychostimulant prescribing were satisfactorily explained by these variables.