The effect of Valium on coronary blood flow was studied. In 18 patients undergoing coronary angiography, total and regional myocardial blood flow was determined by the 133Xe wash-out technique. A total of 39 pre- and 39 post-Valium injection wash-out curves were constructed. Results indicate (99% confidence) that intravenous Valium injection (0.1 mg∕kg body weight) decreases myocardial blood flow 12–15%.