Dorsal root ganglion neurons, which express herpes simplex virus (HSV) latency-associated transcript (LAT) during experimental latent infection, were investigated by in situ hybridization. The number of LAT-positive neurons was determined by examination of ganglion serial sections. In other latently infected mice, the number of ganglion neurons that reactivated HSV antigen after explant culture was determined in serial sections. LAT was detected in 100 percent of ganglia, with an average of 19.5 LAT-positive neurons per ganglion. After explant culture of latently infected ganglia (in the presence of colchicine to decrease spread of reactivated virus), HSV antigen was detected in 94 percent of ganglia, with an average of 13.1 positive neurons in the antigen-positive ganglia. The similar quantities of LAT- and antigen-positive neurons within ganglia support the hypothesis that LAT-positive neurons were the neurons from which HSV was reactivated.