Play has been identified as an indicator of welfare in animals. Opioids have a modulatory effect on play behaviour in rodents, however little is known regarding the central mechanisms involved in play in ruminants. The objective of this study was to determine if opioids are involved in play behaviour in dairy calves. Twenty-four Friesian-cross dairy calves were used in this study: 12 calves were used as experimental animals receiving pharmacological intervention and the other 12 were used as ‘play’ partners. The 12 experimental calves experienced all four treatments in a cross over design: Morphine (MOR) or saline (SAL) administered intravenously 10 min prior to being tested individually (IND) or with a pen-mate (SOC) in an arena test for 20 min. Behaviour was continuously recorded by video during the 20 min test period in the arena to assess the duration of running, and frequency of running and play events. Behaviour was recorded for 6 hours post-treatment in the home pen using HOBO data loggers. Data were analysed using the MIXED procedures of SAS. Calves given MOR tended to perform more social (frontal pushing and mounting) play behaviours in the arena test (2.3 and 1.0±0.66, MOR and SAL respectively, P=0.057) and spent less time lying in the home pen (24.5 and 32.4±3.10%, MOR and SAL respectively, P<0.05) than SAL calves. Calves tested with a familiar pen-mate spent more time running (2.4 and 1.5 ±0.18 min, SOC and IND respectively, P=0.001), but performed fewer play events (bucks, jumps) than calves tested individually (14.9 and 21.3 ±2.37, SOC and IND respectively, P=0.016). Social facilitation appeared to have a greater effect than administration of morphine on play behaviour of calves in an arena test.