Thiopental sodium is known as ultrashort-acting barbiturates and can be employed advantageously for numerous conditions. But thiopental has the side effects of cardiovascular and respiratory systems which has barbiturates and are depend on the dose of thiopental. The side effects are reduced when the thiopental is preceded by a tranquilizer and sedative. In these drugs, benzodiazepines have the minimal effects of cardiovascular and respiratory systems. In this study, the effects of midazolam preanesthetic administration, followed by thiopental anesthetic induction, on cardiovascular system and thiopental induction requirement were studied in 14 mixed breed dogs. Cardiovascular data were recorded baseline, after premedication of saline 0.45 ml/kg or midazolam 0.1, 0.2, 0.4, 0.8 mg/kg, intubation, and 5, 10, 15, 20, 30 minutes after intubation. Extubation, head-up, sternal recombency, standing, and walking recovery times were recorded. The results were summarized as follows; (1) The 0.1, 0.2, 0.4, and 0.8 mg/kg dosages of midazolam insignificantly decreased thiopental dose requirement necessary to accomplish intubation by 6, 20, 21 and 28%. (2) The 0.1, 0.2, 0.4, and 0.8 mg/kg dosages of midazolam insignificantly reduced the times of extubation, head-up, sternal recumbency, standing, and walking recovery. (3) Midazolam was effective in reducing the frequency and duration of arrhythmia after intubation. (4) Heart rates of preanestheic midazolam administraion groups increased after thiopental injection which insignificantly changed smaller than those of control group. (5) Arterial blood pressures did not vary significantly among groups.