The distribution of 99Tcm-HM-PAO is described in normal man. Since our initial report on the clinical potential of this new radiopharmaceutical in 1984, a number of clinical trials are in progress worldwide. As the tracer has been designed to enable the in vivo assessment of the distribution of cerebral blood flow in man, significant interest in this new 99Tcm-labelled compound can be detected not only in the nuclear medicine practice in general, but in the more specialist areas of medicine such as neurology and psychiatry. In this paper we report findings which should aid in the evaluation of this compound in normal individuals.