When Mallory's trichrome stain is used, acinar nuclei of the Harderian gland of Rana esculenta display different affinities for the dye. Some of the orangiophilic nuclei show affinity for aniline blue (blue nuclei). In the Harderian gland of Rana esculenta their number and the intensity of staining with aniline blue may vary during the year. The affinity for aniline blue disappears following digestion of paraffin sections with RNAase, but not with DNAase or trypsin. Furthermore, in vitro incubation with [5, 6-3H]-Uridine shows a selective incorporation by the majority of blue nuclei. Therefore, the affinity for aniline blue is likely due to increased RNA synthesis. The increment of nuclear RNA shown by these methods is supported by the quantitative determination of total RNAs during the resumption (October) and enhancement (May) of secretory activity, when the percentage of blue nuclei of the acinar cells is at its highest levels of the year. The affinity of RNA-rich nuclei for aniline blue, while others are strictly orangiophil, is discussed on the basis of molecular structure of the dyes used in the staining mixture. Mallory's trichrome stain appears to be an useful tool for detecting changes in cell nuclear status.