The effects of chymopapain treatment of the canine intervertebral disc were studied in vivo by monitoring proteoglycan in the nucleus pulposus. Analysis of proteoglycan was carried out by Sepharose CL-4B (Pharmacia Fine Chemicals AB, Stockholm, Sweden) chromatography and electrophoresis. The proteoglycan was degraded to glycosaminoglycans within 1 week after chymopapain treatment. Two weeks later, a proteoglycan smaller than the original appeared in the nucleus pulposus. At 8 weeks after injection, the amount of the newly synthesized proteoglycan, similar in molecular weight to the original, had recovered to about half that of the original, although the new proteoglycan fraction was rich in hyaluronic acid. It was concluded that, following chemonucleolysis with chymopapain, the water-binding capacity of the nucleus pulposus recovered, but that the regenerated nucleus pulposus differed biochemically from the original.