The relationship between second messenger cGMP and human cytomegalovirus (HCMV) replication was investigated. First, the intracellular level of cGMP ([cGMP]i) in HCMV-infected cells was measured. The [cGMP]i increased at early times after HCMV infection, reached maximum level at 12 hr and returned to basal level at 24 hr after virus infection, while [cGMP]i in mock-infected cells remained relatively unchanged. Increasing [cGMP]i resulted in enhanced transcription of HCMV major immediate early gene. For early gene expression, cGMP had varying effect. Expression of 1.2 kb RNA decreased and 2.2 kb RNA increased with increasing cGMP, while 2.7 kb RNA gene expression was not affected. HCMV early genes are regulated by immediate early gene, and the effect of cGMP on the regulatory effect of major immediate early gene on early genes was investigated. In the absence of cGMP, major immediate early gene repressed 2.7 kb RNA gene expression, while 1.2 kb RNA and 2.2 kb RNA early genes were not significantly affected. In the presence of $1;{mu}M$ cGMP, however, major immediate early gene stimulated the expression of three early genes.