We previously reported that the expression of the wheat Cab-1 gene is regulated by an endogenous circadian rhythm and by the photoreceptor phytochrome both in wheat and in transgenic tobacco plants. To define regulatory elements necessary for the circadian rhythm-regulated Cab-1 gene expression, we now analysed the fluctuation of steady-state mRNA levels in a series of 5' deletion mutants in transgenic tobacco plants. We found that the expression of a deletion mutant containing 211 bp upstream sequence still exhibited circadian rhythm. Furthermore we show that an enhancer-like sequence of the Cab-1 promoter (from -357 to -90) can endow a chimaeric gene consisting of a truncated 35S promoter (from -90 to +8) and the bacterial beta-glucuronidase (GUS) gene with circadian clock-regulated gene expression. Finally we demonstrate by nuclear run-off experiments that the transcription rates of the Cab genes in wheat oscillate in a rhythmic manner, with a periodicity of approximately 24 hours. Consistent with our previous findings these results (i) indicate that the expression of the wheat Cab-1 gene is regulated mainly at the transcription level and (ii) identify a short promoter region between -211 and -90 that is responsible for the circadian clock-regulated gene expression.