The direct effects of hydrostatic pressure on matrix synthesis in articular cartilage can be studied independently of the other factors that change during loading. We have found that the influence of hydrostatic pressure on incorporation rates of ³⁵SO₄ and [³H]proline into adult bovine articular cartilage slices in vitro depends on the pressure level and on the time at pressure. Pressures in the “physiological” range (5–15 MPa) applied for 20s or for 5 min could stimulate tracer incorporation (30–130%) during the following 2 h, but higher pressures (20–50 MPa) had no effect on incorporation rates. The degree of stimulation in cartilage obtained from different animals was found to vary;​ in some animals none was seen. Stimulation also varied with position along the joint. Physiological pressures (5–10 MPa) applied continuously for the 2-h incubation period also stimulated incorporation rates, but pressures >20 MPa always produced a decrease that was related to the applied pressure and that was reversible. These results suggests that the hydrostatic pressure that occurs during loading is a signal that can stimulate matrix synthesis rates in articular cartilage.