To compare the histological characteristics of the rat aortic nerve with that of the rabbit, the left aortic nerve of the rat aged 84 days was examined by electronmicroscopy. The numbers of the myelinated and unmyelinated nerve fibers (MF and UMF) were 64 and 124, respectively. The figure of MF agrees with Brown's report (Circ. Res., 39(4), 488-496, 1976). The proportion of the myelinated fibers to all fibers was 34%. The average diameter of the axons was 1.10 ± 0.33 mm in the MF group and 0.38 ± 0.11 mm in the UMF group, and the pattern of the frequency distribution of axon diameters was bimodal, with two peaks corresponding to the MF and UMF groups. The average myelin thickness of MF was 0.33 ± 0.10 mm. In our previous studies, the numbers of MF and UMF of the left aortic nerve in the rabbits (n = 4) aged 56 days were 492 ± 79.5 and 1005 ± 133, respectively, and the proportion of the MF to all fibers was 33%, and the frequency distribution of axon diameters showed a bimodal pattern. In conclusion, the fiber composition and the pattern of the frequency distribution of axon diameters in the rat are similar to those in the rabbit, although the numbers of MF and UMF in the rat are less than those in the rabbit.