Clear distinctions in morphology and proteoglycan composition have been described in regions of adult tendon that pass under bone and are subjected to compressive as well as tensional forces. In this study, developing bovine deep flexor tendon from early fetal stages through 6 months of age was examined biochemically and by light and electron microscopy. Longitudinal collagen fibers were seen in the tensional region of tendon throughout development; whereas a well established network arrangement of collagen fibers with wide interfibrillar spaces was seen in the compressed region by 7 months of fetal age. Collagen fibril diameters of both regions increased with age with the mean diameter in tensional tissue always greater than in compressed tissue. Glycosaminoglycan hexosamine content of the tensional region remained low throughout development (approximately 0.2% of dry tissue weight), but increased in the compressed region from 0.4% of dry weight at the 7-month fetal stage, to 1.0% dry weight at 6 months. Keratan sulfate was not detectable in tensional tendon at any age as measured by inhibition ELISA, but was found in increasing quantities in the pressure bearing region of tendon from young calves. Small proteoglycans predominated in both tensional and compressed regions throughout fetal and early neonatal development, and were of both PG I (biglycan) and PG II (decorin) types. Large proteoglycans represented only a small proportion of total proteoglycans in both regions of fetal tendon. By SDS/PAGE analysis, immunoreactivity, and molecular sieve chromatography, large proteoglycans of fetal compressed tendon were similar to large proteoglycans of adult tensional tendon in that they contained only chondroitin-6-sulfate, with little if any KS, and appeared to be slightly smaller than cartilage large proteoglycans.(ABSTRACT TRUNCATED AT 250 WORDS)