Transcript accumulation patterns for six defence response genes in oat inoculated with an incompatible isolate of Puccinia graminis f.sp. avenae (Pga-1H) or an isolate of an inappropriate forma specialis, P. graminis f.sp. tritici (Pgt-8D), were compared to the occurrence of hypersensitive cell death (HCD) and inhibition of colony elongation. HCD here occurred at 4-6% of infection sites by 36 h after inoculation (AI) increasing to only 26-32% of sites at 72 h AI, whereas colony growth was completely or partially inhibited at 42-48 h AI, indicating that HCD was not a major factor in growth inhibition. By 24-30 h AI, transcripts of three genes accumulated preferentially in the incompatible or inappropriate interactions: 3-phosphoglycerate kinase (3-PGK), an enzyme of glycolysis, possibly associated with early respiratory increase; phenylalanine ammonia-lyase (PAL), related to increased phenylpropanoid biosynthesis, and a gene of unknown function which hybridized to a cDNA, pCRL120, previously obtained from the incompatible interaction. These transcripts accumulated before HCD or fungal growth inhibition, implicating activation of the three genes in early events leading to resistance. By 36 h AI, at the time HCD was beginning to occur, transcripts of a gene for the glucose-regulating protein 94 (GR P94) began to accumulate in response to isolate Pga-1H. Finally, at 42-48 h, in association with HCD and colony growth inhibition, transcripts of genes for a pathogenesis-related protein (PR-1) and for thaumatin-like protein (tlp) accumulated. The results indicate sequential activation of genes, beginning with 3-PGK, PAL and the gene corresponding to pCRL120, followed by GR P94, and ending with PR-1 and tlp.