The aim of this study was to investigate the role of age in the hormonal response to opiate anaesthetic fentanyl. In 90 patients undergoing aortocoronary bypass, 59.6 +/- 9.2 years mean age, 35-81 age range, prolactin (PRL), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH), luteinizing hormone (LH), human growth hormone (HGH), insulin-like growth factor I (IGF I), glucagon and insulin were measured in venous blood samples drawn from fasting patients immediately before, at 8 h in the morning, and 60 min after the induction of anaesthesia with 30 microg/kg intravenous fentanyl bolus, 30 min after a second 7 microg/kg fentanyl bolus. Results showed a higher 60 min PRL peak in older, >65 years, in respect to younger, < or =50 years, patients (57.6 +/- 23.3 vs. 40.6 +/- 13.8 microg/l, P<0.005), with a significant upward trend with age across the entire age span (r=0.32; P<0.002), while no difference by age was found for the basal concentrations. No differences were found between the respective basal and 60 min concentrations for the other hormones investigated. As expected, differences by age were found for FSH, higher in >65 and in 51-65-year-olds than in younger patients (for the basal values, respectively, P<0.02 and P<0.05); IGF I was lower in >65 in respect to < or =50 (P<0.02) and to 51-65-year-old patients (P<0.05), with a significant negative correlation with age (r=-0.33; P<0.005). The study shows an age related increase of PRL concentrations after fentanyl administration. It may be due to the reduction of the hypothalamic dopaminergic tone with aging. IGF I levels have been confirmed to be inversely correlated with age.