The effect of cordycepin, an inhibitor of mRNA synthesis, on the hypersensitive host cell death response (HR), was tested with epidermal tissues from barley coleoptiles containing the Mla gene for resistance, inoculated with incompatible race 3 of Erysiphe graminis f.sp. hordei. In untreated control tissues, HR occurred 18-30 h after inoculation in 61-91% of cells with haustoria. The percentage of HR was strongly reduced by cordycepin at 10-31 μm but only if applied 4 h after inoculation or before; not if applied 8 h or later. Effective cordycepin treatments had no inhibitory effect on the development of germ tubes or appressoria and usually increased the development of haustoria and hyphae. The results indicate that mRNAs required for the HR are synthesized in the host within the first 0-8 h after inoculation and that mRNA synthesis is not required thereafter. The timing of protein synthesis required for the HR in the host could not be determined because the inhibitors of protein synthesis used, blasticidin S and cycloheximide, inhibited parasite development at concentrations required to inhibit the HR.