The purpose of this study was to determine some of the morphological and biochemical effects of sodium morrhuate injections into intact rabbit patellar tendons and Achilles tendons. The effects of one, three, and five 100 microliters injections of sodium morrhuate on tendon circumference, cell content, collagen fibril diameter, collagen-proteoglycan relationships, water content, amino sugar content, and hydroxyproline content were investigated over periods of 1, 4, and 9 weeks. In general, sodium morrhuate injected tendons were larger in diameter and contained more cells, smaller collagen fibrils, increased water and amino sugar content, and reduced hydroxyproline content compared with their contralateral controls. As a sclerosing agent, sodium morrhuate appears to mimic the early stages of an injury-repair sequence when injected directly into intact tendons. Whether sodium morrhuate may hasten repair responses or improve joint laxity remains to be determined.