1. Developing tail tendons from rats (19-day foetal to 126 days post partum) were examined by electron microscopy after staining for proteoglycan with a cationic copper phthalocyanin dye. Cuprolinic Blue, in a "critical electrolyte concentration" method. Hydroxyproline was measured on papain digests of tendons, from which glycosaminoglycuronans were isolated, characterized and quantified. 2. Mean collagen fibril diameters increased more than 10-fold with age according to a sigmoid curve, the rapid growth phase 2 being during 30-90 days after conception. Fibril periodicities were considerably smaller (50-55 nm) in phases 1 and 2 than in phase 3 (greater than 62 nm). 3. Dermatan sulphate is the main glycosaminoglycuronan in mature tendon. Chondroitin sulphate and hyaluronate preponderate in foetal tissue. 4. Proteoglycan was seen around but not inside collagen fibrils. Proteoglycan and collagen were quantified from electron micrographs. Their ratios behaved similarly to uronic acid/hydroxyproline and hyaluronate/hydroxyproline ratios, which decreased rapidly around birth, and then levelled off to a low plateau coincident with the onset of rapid growth in collagen fibril diameter. 5. Dermatan sulphate/hydroxyproline ratios suggest that the proteoglycan orthogonal array around the fibril is largely dermatan sulphate. In the foetus hyaluronate and chondroitin sulphate exceed that expected to be bound to collagen. 6. An inhibiting action of chondroitin sulphate-rich proteoglycan on fibril diameter growth is suggested. 7. The distributions of hyaluronate, chondroitin sulphate and dermatan sulphate are discussed in the light of secondary structures suggested to be present in hyaluronate and chondroitin sulphate, but not in dermatan sulphate.