This thesis describes a prospective, randomized, blinded study to investigate the effects of midazolam as a co-induction drug in combination with propofol, in critically ill dogs. Case enrollment included dogs with a weight ≥ 10 kg and age ranging from 1-15 years, requiring an exploratory laparotomy and classified according to the American Society of Anesthesiologist physical status score (ASA) as III or IV. Dogs were also scored for the Survival Prediction Index (SPI2) to predict their surgical risk. Dogs were allocated to two groups, which included premedication with two separate doses of fentanyl; F1 (2 µg kg-1, IV) and F2 (3 µg kg-1, IV) for instrumentation; then one group was induced with IV propofol, (P: propofol 1 mg kg-1) and the other group with propofol and midazolam (M: propofol 1 mg kg-1, followed by IV midazolam (0.3 mg kg-1), with subsequent propofol (0.25 mg kg-1; q6 seconds) as required to achieve endotracheal intubation. Both groups had anesthesia maintained with isoflurane. Sedation and induction quality was assessed, and dose requirements for induction with propofol were measured and compared between the groups. Selected cardiopulmonary parameters were assessed after F1 and F2, induction, and maintenance with isoflurane with spontaneous and mechanical ventilation during surgical preparation. Measurement times were defined as follows: T0, patients transferred from the intensive care unit (ICU) to anesthesia; T1, 2 minutes after F2; T2, within 5 minutes from induction; T3, patients placed in dorsal recumbency and mechanically ventilated; and T4, surgical preparation. There was no difference in age or weight. Dogs in the M group had a significantly lower propofol induction dose requirement than the P group, associated with similar sedation scores between groups, but a significantly better intubation/induction score in the M group. However, heart rate, arterial blood pressure, respiratory rate, cardiac index, systemic vascular resistance, end-tidal isoflurane concentration and end-tidal carbon dioxide concentration, were similar between groups at all time intervals. The distribution of ASA and SPI2 scores for the dogs was similar in both groups, but there was no correlation between ASA and SPI2 scores. In conclusion, this study demonstrates that co-induction with midazolam improves endotracheal intubation and the induction phase. Despite a reduction in the induction dose of propofol, midazolam co-induction did not improve cardiopulmonary function in critically ill dogs, when compared to a higher dose of propofol alone.