The early effects of human growth hormone (hGH) were investigated by knemometry, a recently described technique of measuring lower leg length. In 14 growth hormone deficient children, the mean increase in the lower leg length was 0.47 +/- 0.10 (SE) mm within 24 h of a single intramuscular injection of 4 IU hGH. This increase occurred after about 80% of the single injections and was highly significant (p less than 0.01). 48 h after an injection, a slight but not significant reduction of the lower leg length was observed (-0.15 +/- 0.09 [SE]). These effects led to a marked zig-zag pattern of growth when hGH was given thrice weekly.