1. We have examined the hypothesis that the raised plasma renin activity in patients with malignant hypertension without an underlying cause is the consequence of expression of a duplicate renin gene. 2. DNA extracted from leucocytes of patients with malignant hypertension and of normotensive controls was digested with the restriction endonuclease PstI and hybridized with a radioactively labelled human renin complementary DNA probe. As an internal control the DNA was concurrently hybridized with a human c-myc protooncogene probe. 3. The signals for each subject from the two probes were quantitatively compared by densitometry. 4. There was no evidence of duplication of the renin gene in the patients with malignant hypertension.