OBJECTIVE: To evaluate the effects of local anaesthesia with lidocaine for castration of horses under intravenous anaesthesia. STUDY DESIGN: Prospective, randomized, blinded clinical trial. ANIMALS: Fifteen equidae, scheduled to undergo castration under total intravenous anaesthesia, were randomly distributed in two groups. One group received lidocaine injections (group L: two ponies, four horses, two donkeys) and the other received saline (group S: two ponies, three horses, two donkeys). METHODS: Behaviour, heart rate (HR) and respiratory rate (f(R)) were evaluated prior to anaesthesia. Body mass was measured using an electronic scale and testicular volumes were estimated. The animals were anaesthetized with acepromazine intramuscularly and romifidine intravenously followed 10 minutes later by ketamine. Following romifidine administration lidocaine or saline was administered subcutaneously along the incision line and by intratesticular and intrafunicular injection. Based on clinical observations (movement, f(R), and cranial nerve reflexes) incremental intravenous doses of ketamine and romifidine were administered. HR, f(R), oscillometric mean arterial blood pressure (MAP), duration of surgery, movement and additional doses were recorded. Surgical conditions were assessed using a visual analogue scale (VAS) and a simple descriptive scale (SDS). Recovery was assessed by two assistants, unaware of treatment, acting separately using a VAS and a SDS. Group means were compared using Mann-Whitney and Wilcoxon tests and the Kruskal-Wallis signed rank test for matched pairs used to compare groups at different points (p < 0.05). RESULTS: The number (median, range) of incremental doses (4 [1-5] compared to 1.5 [1-4]) and movements (1 [1-5] compared to 0 [0-1]) were higher (p = 0.01 for both) in the control group than in the lidocaine group. Groups were similar for other recorded variables. CONCLUSIONS AND CLINICAL RELEVANCE: These results show the effectiveness of lidocaine used as a local anaesthetic adjunct to intravenous anaesthesia in horses undergoing castration.