In a morphometric analysis of rat tail (RTT) and extensor digitorum longus (EDLT) tendons in rats from birth to 12 weeks of age it was found that the volume fraction of tenoblasts and the surface density of their plasma membranes fell sharply. The collagenous extracellular fraction rose in a reciprocal manner over the same period. The volume fraction of the tenoblast rough endoplasmic reticulum stayed more or less constant from newborn to 12 weeks. The collagen fibril diameters displayed a sharp unimodal distribution at birth, but at 12 weeks became bimodally distributed and of greater range, especially in RTT. Generally, observations were similar to data previously published in qualitative studies, but there were small differences between RTT and EDLT which may be accounted for by differences in function.