The accumulation of Cat3 catalase mRNA in the green leaves of maize (Zea mays) exhibits an unusual circadian rhythm. The steady-state level of the Cat3 transcript varies dramatically over the day in seedlings grown on a 12-hr photoperiod. Low or undetectable levels of the Cat3 mRNA are found late in the dark and early in the light period, while the transcript accumulates to high levels late in the light and early in the dark period. This dramatic diurnal variation in mRNA level does not occur with the maize Cat1 or Cat2 catalase transcripts. The diurnal fluctuation in Cat3 mRNA persists when the seedlings are transferred to continuous light or darkness, which indicates the influence of a circadian rhythm. The lack of influence of red and/or far-red light on this diurnal variation in transcript level indicates that the circadian rhythm is not regulated by phytochrome. Run-on transcription assays show that changes in the rate of Cat3 gene transcription closely parallel the observed changes in steady-state mRNA levels. This circadian rhythm in Cat3 gene expression and the resulting high levels of mRNA which accumulate late in the light period suggest that the catalase 3 (CAT-3) isozyme is being synthesized for accumulation in the dark period. This, together with the high level of Cat3 mRNA and CAT-3 protein accumulation in dark-grown maize shoots, suggests that the activity of the CAT-3 isozyme might be associated with a metabolic process important in shoot cells in the dark.