Five clinically healthy Thoroughbred geldings were injected with Freund's adjuvant 3 times to induce a chronic inflammatory response. Blood was collected at various times before and after adjuvant administration. Clinical responses (rectal temperature and general demeanour) were also monitored. Adjuvant injection induced increases in rectal temperature and plasma fibrinogen concentration (maximum levels measured were mean ± s.d. 39.7 ± 0.5°C and 8.2 ± 03 g/l, respectively), indicative of an inflammatory response. A mild clinical depression was also observed in the horses for 24 h after the first injection of adjuvant only. Plasma Cortisol levels decreased significantly from control levels of mean ± s.d. 187.7 ± 24.3 nmol/l to a minimum of 80.2 ± 22.1 nmol/l (P<0.01) 9 days after the first injection of adjuvant. Conversely, plasma insulin levels increased after the first injection of adjuvant to a maximum (96.7 ± 15.2 iu/ml; P<0.01) 12 days later, while plasma glucose concentrations tended to decline. A control group of horses to rule out contemporary environmental influences on the physiological and biochemical indices measured was not included in this study. The results show that chronic inflammation in the horse depressed resting plasma Cortisol concentrations.