In a previous study that used butorphanol in pigs before castration performed under isoflurane anaesthesia, severe adverse effects were recorded. As in pigs, this has not been reported before, we aimed to investigate the effects of butorphanol in piglets. In this study ten 27 days old piglets were randomly allocated to receive either 0,2 mg/kg butorphanol (group B) or saline 0,9% (control group C) intramuscularly. Their behaviour was as-sessed for 60 minutes by two independent observers from videotapes. Two to 15 minutes after application, piglets in group B showed restlessness, distress and excessive vocalisation. Locomotor activity was increased, the piglets laid down considerably less frequently (p = 0,034) and for shorter time periods (p = 0,0014) during the first 40 minutes compared to group C. Group C animals slept most time of the experiment (45,1 ± 2,9 minutes in group C vs 12,7 ± 2,9 minutes in group B, p < 0,0001). After receiving butorphanol, pigs showed jumping against the wall (mean 1,2 times per minute during the first 30 minutes in group B vs 0 times per minute in group C, p = 0,0011). In group B, one animal pressed its head against the wall and four animals showed severe panting and gasping. Thirty minutes after butorphanol application piglets became hyperthermic (41 ± 0,7°C group B vs 39,6 ± 0,3°C group C, p = 0,0075). The results of this study show that 0,2 mg/kg butorphanol intramuscularly induces severe side effects in pain-free piglets that are similar to those reported following opioid administration in other species.