1. The rate of smooth muscle cell proliferation in age-matched 1-4-week-old spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats was compared using autoradiography.

2. Labelling index, defined as labelled cells/sum of labelled and unlabelled cells × 1000, was obtained from perfusion-fixed superior mesenteric and large mesenteric arteries.

3. In the large mesenteric arteries, the smooth muscle cell labelling indices were similar between the SHR and WKY at all age groups, except at 1 week of age when the smooth muscle labelling index was higher in the SHR.

4. In the superior mesenteric arteries, labelling indices were similar between the rat strains at all age groups.

5. We conclude that, in the SHR, a rapid proliferation of smooth muscle cells in the large mesenteric arteries occurred during the first week of life. This resulted in a higher number of smooth muscle cell layers in the media of muscular arteries.

6. The increased proliferation may play a role in the subsequent development of hypertension in the SHR.

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