Castration is a routine procedure for male farm animals. The ethics of castration are widely debated because the procedure may potentially result in pain and distress. The indications for early castration in farmed guanacos are: 1) prevention of aberrant behaviour in human-imprinted males, 2) elimination of inter-male aggression, so that males may be housed together or with females, 3) avoidance of accidental breeding (Fowler, 1998). In addition, it has been argued (Molony and Kent, 1997) that to study the pain response to castration is ethically acceptable as the overall welfare of the animal is improved by the procedure. Guanaco farming has been recently established in South America, and there is therefore little information available on how guanacos respond to castration. The purpose of this study was to assess pain in farmed guanacos. The hypothesis was that pain experienced by guanacos due to castration may be displayed in behavioural and physiological changes, as has been reported in lambs, calves and piglets. The study was carried out in 2001 on four-month-old farmed male guanaco. Changes in plasma cortisol concentration and the frequency of specific daytime behavioural postures and activities following castration are reported.