Feulgen deoxyribonucleic acid (F-DNA) microspectrophotometry was evaluated as a potential tool for quantification of herpes simplex virus type 1 (HSV-1) and 2(HSV-2) DNA synthesis in single cells. Since HSV DNA synthesis has been extensively studied using incorporation of radioactive precursors into viral and cellular DNA, microspectrophotometric measures were correlated with biochemical data obtained using tritium-labeled thymidine ([3H]TdR). It was established that: (i) viral-induced increaae in F-DNA can be cytophotometrically detected between 1 and 6 h postinjection (p.i.), which corresponds to the initial incorporation of [3H) TdR into viral DNA; (ii) peak F-DNA levels occurred 8 h p.i., which supported cytological observations of a more rapid development of an inclusion body in HSV-2-infected nuclei. Despite the fact that F-DNA cytophotometry is unable to distinguish between cell and viral DNA, the overall study supports the existence of a good correlation between data obtained using microspectrophotometric and conventional isotope methods. Furthermore, cytophotometry complements biochemical evaluations in that it permits analyses of DNA changes on a single-cell basis or the detection of infection in very small numbers of cells.