Ultrasound has been applied therapeutically to accelerate connective tissue healing although there is little direct scientific evidence to support its use. This investigation was conducted to determine the effects of ultrasound on the rate of collagen synthesis and cell proliferation using cultured fibroblasts derived from Achilles tendons of neonatal rats. Ultrasound (intensity = 0.4 W.cm-2; frequency = 1 MHz) was applied to experimental cells growing as monolayers in culture flasks. Ultrasound had no effect on the rate of collagen synthesis by control fibroblasts over a period of 9 d. The addition of vitamin C to culture media stimulated collagen synthesis to the same extent in both control and ultrasound-treated cultures. Partial digestion of cell matrices with collagenase (used to simulate injury) resulted in an approximately 20% increase in the rate of collagen synthesis. Synthesis was further increased with ultrasound treatment (50-67%). For example, after a single ultrasound treatment, the rate of collagen synthesis was 3.0 +/- 0.4 pg.micrograms-1 DNA.h-1 in cultures treated with collagenase, compared with 1.8 +/- 0.3 pg.micrograms-1 DNA.h-1 in collagenase-treated cultures not treated with ultrasound and 1.4 +/- 0.3 pg.micrograms-1 DNA.h-1 in controls. Ultrasound applied to preconfluent cultures resulted in significant increases in the rate of thymidine incorporation and DNA content. Three daily ultrasound treatments caused a 100% increase in the rate of thymidine incorporation and a 28% increase in DNA content. The results indicate that ultrasound stimulates collagen synthesis in tendon fibroblasts in response to an injury of the connective tissue matrix and that ultrasound stimulates cell division during periods of rapid cell proliferation.