Geometric measurements were made of the nuclei of the muscle cells of the tunica media of human intracranial arteries. The nuclei measured 37 +/- (SD) 6.4 micron in length, and 2.0 /+- (SD) 0.83 micron average width and have a number density of 10(5) nucei/mm(3) (number per unit volume of tunica media). From the work of others it was known that the nuclei are centrally located within the cytoplasm of the muscle cell; because of this we have used the nuclei as indicators of alignment of the muscle cells themselves, making measurements from light micrographs of arterial sections stained with haematoxylin and eosin. On average, the nuclei were oriented at an angle that was not significantly different from a zero-degree pitch for the five sections analyzed in detail.