The active metabolite of morphine, morphine-6-glucuronide (M6G), may have fewer unwanted effects than morphine. We randomly allocated 144 women to receive either M6G or morphine as part of general anaesthesia for day case gynaecological laparoscopy. The incidence of nausea, vomiting, pain, sedation and skin rash, and severity of nausea, pain and sedation after surgery were recorded by direct observation in hospital, and by questionnaire until the next morning. Compared with the M6G group, patients who received morphine were more likely to report nausea in the first 2 h after surgery (odds ratio 2.9, CI 1.31-6.21) and to suffer it with greater severity. During the same time period, they were more likely to vomit and feel sleepy, but the intensity of pain and use of rescue analgesics were similar in both groups. The incidences of nausea, vomiting and the feeling of sleepiness continued to be greater in the morphine group during and after the journey home. The next morning, patients in the morphine group remained sleepier, but the incidence of nausea was similar for the two groups. M6G appears to have a better toxicity profile than morphine. More efficacy studies are needed to define accurately the analgesic potency of systemically administered M6G.