As an approach to understanding the abnormalities of pro-opiomelanocortin (POMC) gene regulation in human ACTH-secreting tumours, we have analysed the POMC mRNA content of nine such tumours using the Northern blot technique. Most of the tumours and normal human pituitary contained easily detectable quantities of POMC mRNA. The length of this message in most tumours was similar to, or slightly larger than, that in the normal pituitary (1150-1200 bases). Ribonuclease H studies suggested that the origin of any size heterogeneity was a longer poly(A) tail in the tumour RNA. Some tumours, however, expressed a short POMC mRNA (800 bases) which may lack the first two exons of the POMC gene as has been described. A third POMC mRNA size variant (1500 bases) was also seen in low levels in two cases, and as the principal mRNA species in one case. Primer extension and S1 nuclease protection studies suggested that most transcripts in the tumours analysed originated from the conventional promoter, and thus the use of an alternative promoter is not an adequate explanation for the expression of this gene in ectopic ACTH-secreting tumours.