Age-related changes of pyridinoline, a mature cross-link of collagen fibers, in tissues of guinea pigs which cannot synthesize L-ascorbic acid (AsA) were investigated. Male guinea pigs, 2 weeks old, were fed a commercial diet until they were 40 weeks old. Based on the data of body weight gain, it is assumed that guinea pigs require 10 weeks to reach maturity. The content of AsA in serum, adrenals, and liver of the animals increased with growth, then decreased after 8-10 weeks. The pyridinoline content in cartilage and tendon collagen was very low in 2-week-old animals, whereas it increased markedly with growth. After 10 weeks, the pyridinoline tended to decrease in cartilage collagen; however, it was not changed in tendon collagen. The age-related changes of pyridinoline content in guinea pigs cartilage were similar to those of humans--increasing with growth and decreasing after adolescence.