Sixteen colts were premedicated with acepromazine and anaesthesia was induced with detomidine and ketamine. Ponies were randomly allocated to receive halothane (HAL) or infusion of detomidine, ketamine and guaiphenesin (DKG) to maintain anaesthesia. Heart and respiratory rate, ECG, mean arterial blood pressure (MABP), cardiac index (CI), blood gases and plasma cortisol, ketamine and guaiphenesin were measured. Surgical castration took place between 45 and 75 min and anaesthesia lasted 90 min. MABP with DKG was significantly higher than with HAL, and, with HAL, MABP increased from pre-surgery (64 +/- 6 mmHg) to mid-surgery (80 +/- 5 mmHg) but did not change with DKG. At 30 min, CI was similar in both groups (57 +/- 7 ml/kg bwt/min); it decreased during surgery with HAL and remained low, but it increased slightly with DKG, and was higher than with HAL at 60 and 90 min. Plasma cortisol decreased in both groups until 40 min then increased with HAL only during surgery. Ketamine concentration reached a plateau (1.3-1.8 microg/ml) between 20 and 90 min and guaiphenesin concentration between 60 and 90 min (99-101 microg/ml). Recovery was generally smooth in both groups. This study demonstrated that during HAL the increase in blood pressure associated with surgical stimulus is accompanied by decreased CI; this did not occur during DKG which is likely to lead to better tissue perfusion than HAL. The adrenocortical activity seen during HAL was absent during DKG which may result from pituitary depression, analgesic effects of total intravenous anaesthesia (TIVA) or better perfusion.