The Harderian gland is a tubule-alveolar gland found within the orbit, on the posterior aspect of the eyeball. In mice, it is composed almost exclusively of secretory tubules and alveoli. The Harderian glands of transgenic mice, carrying the activated N-ras oncogene under the transcriptional control of the mouse mammary tumor virus long terminal repeat promoter (MMTV-LTR), were examined and compared to those of normal mice. Thirty transgenic mice provided by A. Pellicer (NYU) and 30 normal mice were examined in this study. Harderian glands were dissected, immersed in a formalin-based fixative, and embedded in paraffin. The sections of these glands were studied via histological techniques. Our results show that proliferative alterations in the Harderian glands of these transgenic mice are present even in the youngest animals. Such alterations correspond to different tumoral evolution stages, ranging from hyperplasia to wide tissue destruction. In the most advanced situations, these changes are accompanied by a glandular hypertrophy. Our results suggest a very high tumoral incidence in the Harderian glands of transgenic mice compared to normal mice. Tumors appear spontaneously in some areas, but not at the same time in the whole gland.