A convenient, Agrobacterium-mediated transient expression assay has been evaluated for rapid analysis of plant promoters and transcription factors in vivo. By simple infiltration of Agrobacterium cells carrying appropriate plasmid constructs into tobacco leaves in planta, reproducible expression assays could be conducted in as little as 2-3 days without using expensive equipment (e.g. biolistic gun or electroporation apparatus) or complicated procedures (e.g. preparation of protoplasts). Biotic and abiotic treatments could be applied to the intact plant to examine their influence on promoter activity and gene expression. Using this method, we have tested the stress-responsive as-1 and heat shock elements, yeast GAL4 transactivation system, two promoters of pathogenesis-related (PR) genes as well as a heat shock promoter. Through deletion analyses of tobacco PR1a promoter and a novel myb1 promoter, we have also successfully identified the cis-regulatory regions in these promoters that are responsive to salicylic acid treatment or tobacco mosaic virus infection. Together, our results demonstrate that Agrobacterium-mediated transient expression is a simple and efficient method for in vivo assays of plant promoters and transcription factors.