Considerable variation has been observed between studies investigating the milk yield (MY) response (MYR) to exogenous growth hormone (GH) in dairy cows. Daily injections of 25 mg/day of the recombinant product caused a MYR of over 10 kg/d (Bauman et al. 1985), a level of response rarely repeated since. Other studies using apparently similar doses and management regimes have reported no substantial effect of the hormone on MYR (e.g. Hof et al. 1991). While these studies are extreme examples, it is clear that variability in MYR has occurred between trials. In contrast, relatively little information has been published concerning variation in the MYR between treated cows. Indeed, the notion of individual variation in the MYR to GH remains contentious: Bauman (1992) suggested that the variation in the MY of GH treated cows was similar to that of control cows, and that variation in the MYR between individuals does not exist. The object of this experiment was to investigate the extent of the individual variation of the MYR to GH in dairy cows, and to relate this to changes in plasma concentrations of hormones and metabolites.