Herpes simplex virus (HSV) mutants defective for thymidine kinase expression (TK-) have been reported to establish latent infection of sensory ganglia of mice, in that HSV latency-associated transcript is expressed, but to be defective for reactivation. In the present study, the mechanism of defective reactivation by TK- HSV was investigated. Latent infection established by each of three reactivation-defective HSV type 1 mutants was studied. Reactivation in explant culture was markedly enhanced by the addition of thymidine (dTdR) to the explant culture medium. Without added dTdR, reactivation occurred in 0 of 32 ganglia, while when dTdR (200 microM) was present, reactivation occurred in 32 of 37 ganglia (86%). Reactivation was minimal or did not occur after treatment with other nucleosides; specificity for dTdR would suggest the importance of dTdR nucleotide levels rather than more general nucleotide pool imbalance. Enhanced reactivation by dTdR was dose dependent and was blocked by acyclovir. While some degree of inhibition of TK- HSV by acyclovir may be expected, the complete block of dTdR-enhanced reactivation was unexpected. This result may suggest that HSV is particularly vulnerable during initial reactivation events. The mechanism of dTdR-enhanced reactivation of TK- HSV was further evaluated during in vivo infection by TK- HSV. For mice infected with TK- HSV, virus was undetectable in ganglia 3 days later. However, for mice infected with TK- HSV and treated with dTdR, virus was readily detected (2.8 x 10(3) PFU per ganglion). This result suggested that in vivo treatment with dTdR enhanced replication of TK- HSV in ganglion neurons. In turn, this suggests that in latently infected ganglia, dTdR-enhanced reactivation of TK- HSV occurred as a result of viral replication in neurons following initial reactivation events.