1. Cardiac hyperplasia in newborn spontaneously hypertensive rats (SHR) has been noted by our group as well as by other investigators. The present study was designed to establish whether early (neonatal) hyperplasia is confined to the heart or is a generalized phenomenon in this hypertensive model.

2. The ventricles, kidney and liver of newborn SHR, Wistar and Wistar-Kyoto (WKY) rats were analysed for their protein and DNA content.

3. Total organ weight and the organ/body weight ratio of the heart and kidney but not of the liver were significantly greater in the SHR than in the control rats, irrespective of total body weight.

4. The higher relative DNA content (per 100 mg of tissue or 100 g body weight) indicated that hyperplasia rather than hypertrophy was responsible for the enlarged heart as well as the kidney of newborn SHR.

5. The cause of this selective cardiac and renal hyperplasia is not yet known: it may be due to putative ‘haemodynamic growth stimuli’, an intrinsic genetic abnormality of cells in the heart and kidney, circulating growth-promoting factors, or innervation.

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