Clonidine is a specific alpha-2-adrenoreceptor agonist that stimulates growth hormone (GH) release in animals and humans. This drug was used to study the GH and prolactin (PRL) secretory response in dairy cows and heifers. An i.v. infusion of 10 micrograms/kg body weight induced GH release to a peak concentration after 30-60 min, while 2 micrograms/kg had no effect on GH secretory patterns. Plasma PRL decreased significantly (P < 0.01) starting 15-60 min after both doses of clonidine, this effect lasting up to 6 h. Clonidine significantly lowered plasma insulin (P < 0.01) and raised plasma glucose (P < 0.01). The changes in plasma GH, PRL, insulin and glucose differed significantly between doses, the 10 micrograms/kg dose being more effective (P < 0.01). The results of our investigation in dairy cattle provide evidence of (i) an increase in GH release after 10 micrograms/kg clonidine; (ii) a concomitant decrease in PRL secretion, hence GH and PRL secretion in cattle appear inversely controlled; (iii) a significant difference between the effects of the 2 and 10 micrograms/kg doses and (iv) no relationship between the changes in plasma GH and PRL after clonidine and plasma hormone levels before treatment.