ICR mice were inoculated intracamerally with McKrae strain herpes simplex virus (HSV) followed by intraperitoneal injection with 3H-thymidine. Infected mice were sacrificed after 3 or 4 days and the eyes, trigeminal ganglia (TG) and superior cervical ganglia (SCG) were embedded in glycol methacrylate, sectioned, and dipped for autoradiography. Light microscopy revealed silver grain labeling over neurons in the ipsilateral retina, TG and SCG of infected animals. No label ing of neurons was noted in the contralateral TG or SCG. Since the DNA of mature neurons does not replicate, we interpret these labeled neurons to represent cells with active replication of HSV. This technique allows the study of HSV infection of the nervous system with excellent tissue preservation. Furthermore, it may be used to distinguish those neurons with intrinsic viral synthesis from those harboring virus synthesized at a distant site with subsequent intracellular spread.