The intervertebral disc is the largest avascular tissue in the body. Its metabolism is mainly anaerobic, and thus lactate is produced at a significant rate. As a result the lactate concentrations in the center of the disc may be 8–10 times as high as in the plasma. The pH in the disc center is thus acidic. Because low values of pH are known to affect proteoglycan synthesis in other cartilages, the authors measured the effect of lactate levels and pH on 35S-sulphate and 3H-proline incorporation rates in the nucleus of bovine coccygeal discs and in human disc obtained during percutaneous nucleotomy. The maximum incorporation rate occurred at pH 7.2-pH 6.9. Here the rate was 40–50% greater than at pH 7.4. Below pH 6.8 the rate fell steeply, more so for sulphate than for proline. At pH 6.3 the sulphate incorporation rate was around 20 percent that at pH 7.4. The results indicate that proteoglycan synthesis rates in particular are sensitive to extracellular pH, and that the peak rate occurs around the level of pH seen in vivo. Factors that cause lactate levels to rise, such as a fail in O2 levels as the result of smoking or vibration (Holm and Nachemson, 1988) could lead to a fall in proteoglycan synthesis rates, ultimately leading to a fall in proteoglycan content and to disc degeneration.