This study was designed to determine whether juvenile growth is accompanied by changes in the local influence of nitric oxide (NO) or prostaglandins on arteriolar tone. In vivo microscopy was used to study proximal arterioles in the spinotrapezius muscle of rats 4-5 wk (weanling), 7-8 wk (juvenile), and 11-12 wk (mature) of age. From 4 to 12 wk of age, arterioles underwent an increase in resting diameter (from 31 ± 2 to 49 ± 2 µm) and volume flow (from 7 ± 1 to 10 ± 1 nl/s) but a decrease in resting wall shear rate (from 1,901 ± 150 to 748 ± 50 s¹). NO synthase inhibition with N^G-monomethyl-L-arginine (L-NMMA) had no effect on arteriolar diameters in weanling rats but reduced diameters by 14 ± 4% in juvenile rats and by 13 ± 4% in mature rats. Cyclooxygenase inhibition with meclofenamate reduced arteriolar diameters by a similar amount (13 ± 4 to 18 ± 3%) in all age groups. There were no age-related differences in arteriolar responsiveness to locally applied sodium nitroprusside or prostaglandin E₂. Arteriolar responsiveness to ACh was also similar in all groups, but the L-NMMA-sensitive portion of this response was smaller in mature rats than in weanling rats. Elevation of flow-related shear stress caused arteriolar dilation in juvenile rats but not in weanling rats. These findings suggest that arteriolar smooth muscle responsiveness to NO or prostaglandins does not change during juvenile growth and that basally released vasodilator prostaglandins exert a constant influence on arteriolar tone throughout this period. Basal NO activity also modulates arteriolar tone in juvenile and mature rats but not in weanling rats. In contrast, agonist-stimulated NO release is prominent in weanling and juvenile rats but somewhat decreased in mature rats, where cyclooxygenase products also contribute to ACh induced dilation.