Summary The acute pain produced by bloodless castrators was studied by comparing the behavioural and plasmacortisol changes in groups ( n=8) of 3-week-old Dorset cross lambs after castration with a 22 cm (9″) Burdizzo, a new power assisted castrator and by a combined method using a Burdizzo and elastrator ring. The time spent in abnormal postures (52–58 min) and the peak cortisol response (110–120 mmol l −1) were similar for the three methods, although the powered castrator produced a more sustained response. The Burdizzo method halved the incidence of active behaviours compared with the powered castrator and combined methods (16 versus 30, 32 counts). Intratesticular local anaesthetic administered 2 min before the Burdizzo castrator and combined method, or intramuscular injection of the non-steroidal anti-inflammatory drug diclofenac, 20 min before the application of a Burdizzo, significantly reduced the peak plasma cortisol response to 80 nmol l −1. Diclofenac also significantly reduced the time spent trembling or in abnormal postures.