Broilers from 2 trials which were blood sampled by cardiac puncture at 4 and 8 weeks had lower 8 week body weight than corresponding non-blood sampled birds (Trial 1, P < 0.02; Trial 2, P < 0.05). In another experiment, S.C.W.L. replacement pullets blood sampled by cardiac puncture at 4, 8, 12, 16 and 20 weeks of age had lower body weights at all ages than non-blood sampled birds. The effect was greatest at 20 weeks of age (P < 0.05).