To compare the effects of two premedication regimens on cardiorespiratory variables, sedation, and anxiety in patients scheduled for coronary artery bypass graft (CABG) surgery.This was a prospective randomized, double-blind clinical trial. Sixty-eight patients were monitored for 1.5 hr before and 2.0 hr after premedication with lorazepam (0.03 mg.kg-1 sl), morphine (0.15 mg.kg-1 im), and perphenazine (0.05 mg.kg-1 im) [Group 1], or with lorazepam (0.03 mg.kg-1 sl) and saline (1.5 ml im) [Group 2]. All were continuously monitored with a 12-lead ECG ST monitors, respiratory inductive plethysmography (RIP), digital pulse oximetry, intra-arterial blood pressure, and arterial blood gas analysis. Sedation and anxiety scores were also recorded.The incidence and duration of myocardial ischaemia was low and similar in Groups 1 and 2. Patients in Group 1, but not in Group 2, had a greater number of events (P < 0.04) and duration (P < 0.02) of O2 desaturation; higher PaCO2 (P < 0.001), and more haemodynamic events (P < 0.006) after premedication when compared with baseline. There was no difference in RIP or ECG variables between the two groups. Following premedication, both groups reported reduced anxiety scores and elevated sedation scores (P < 0.01), with sedation greater in Group 1 than in Group 2 (P < 0.01).In CABG patients, premedication with lorazepam provides adequate anxiolysis and sedation, and the addition of morphine and perphenazine results in elevated PaCO2, arterial haemoglobin desaturation, and potentially adverse haemodynamic changes.