Reverse transcription followed by real-time quantitative polymerase chain reaction (RT Q-PCR) is useful for the systematic measurement of plant physiological changes in gene expression. The validity of using 18S rRNA and three housekeeping genes, glyceraldehyde-3-phosphate dehydrogenase, actin, and tubulin, was tested as a reference of RT Q-PCR. Under various growth stages of etiolated seedlings, different cultivars, and various times after UV-irradiation treatment, expression level of 18S rRNA correlated with total RNA suggesting the uniformity of RT Q-PCR efficiencies among samples. Relative expressions of housekeeping genes varied among samples and independently of experimental conditions, up to two-fold, signifying generally constant fraction of mRNA in total RNA. Results indicate 18S rRNA was the most reliable reference gene for RT Q-PCR of total RNA.