Cycloheximide, which had already been shown to inhibit protein synthesis in Earle's L cells (mouse fibroblasts) without having any effect on the multiplication or protein synthesis in Chlamydia psittaci (strain meningopneumonitis) infecting these host cells, also caused greater than 90% inhibition of deoxyribonucleic acid (DNA) synthesis in L cells after a 3-hr exposure to the drug. L cells infected with the meningopneumonitis agent and treated with cycloheximide were used to follow meningopneumonitis-specific DNA synthesis during intracellular growth of the parasite. The rate at which labeled precursors were incorporated into parasite DNA doubled every 2 hr. The effect of meningopneumonitis infection on L-cell DNA and protein synthesis was investigated in logarithmically growing and in stationary-phase (nondividing) populations of L cells. Host-specific DNA and protein synthesis appeared to be inhibited in infected L cells when compared with logarithmically growing control cells, whereas no inhibition was apparent when the comparison was made with stationary-phase control cells. The maximal amount of protein and DNA synthesis that occurred in meningopneumonitis-infected L cells was equal to the amount of DNA and protein synthesized in logarithmically growing, uninfected L cells. A possible explanation of these results is given.