We studied the levels of collagen type I and type III in the knee joints of rats immobilized for periods of 2, 4, 16 and 32 weeks and sham-operated controls. The intensities of immunostaining of the anterior and posterior synovial intima, anterior and posterior subintima, and patellar tendon were graded on a scale from 0 to 3. We found higher type I collagen levels in immobilized legs than in sham-operated legs in the anterior subintima after 4 and 16 weeks (mean score 2.1 vs 1.3, 2.2 vs 1.3, respectively) and posterior subintima after 2, 4 and 16 weeks of immobility (2.4 vs 1.7, 2.3 vs 1.5, 2.2 vs 1.3, respectively). Lower type III collagen levels were found in immobilized legs than in sham-operated legs in the anterior synovial intima after 32 weeks (1.3 vs 2.3), and posterior synovial intima and posterior subintima after 16 weeks of immobility (1.4 vs 2.8, 1.2 vs 1.7, respectively). The higher type I collagen levels in the subintima combined with lower type III suggests that the contracture process is marked by fibrosis, not new tissue proliferation. In this respect, contractures differ from granuloma, scar tissue and the pannus of inflammatory arthritis.