Growth was strongly related to growing stock level (GSL) for 5 years after thinning 20-year-old poles on Site Index50 115 land at the Elliot Ranch Plantation in northern California. Five GSL's- basal areas anticipated when trees average 10 inches d. b. h. or more - ranging from 40 to 160 square feet per acre were tested. Periodic annual increment (PAI) in diameter decreased curvilinearly from 0.51 to 0.21 inch, with increasing GSL's from 40 to 160. But only trees in plots heavily thinned to GSL 40 (and briefly in GSL 70) grew faster in diameter after thinning than these same trees grew before thinning. PAI in basal area and net volume rose steadily with increasing GSL9s, reaching 9.0 square feet and 255 cubic feet per acre, respectively, at GSL 160. Apparently, even light thinning reduced volume production, but thinning still is recommended to shorten time until first commercial entry and to promote stand health.