Rat pineal gland contains a 12-lipoxygenase as demonstrated by the enzyme activity, RNA blot analysis and in situ hybridization. Using rats maintained with 12-h dark and light cycles, dynamic changes of the enzyme in pineal gland were examined. When the crude extract of pineal glands was incubated with arachidonic acid and the reaction products were analyzed by reverse-phase HPLC, the glands obtained from rats in the dark showed a higher 12-lipoxygenase activity than those obtained from rats in the light. The pineal 12-lipoxygenase activity decreased after the light was on at 7 o'clock and reached the lowest level around 16 o'clock. Upon Western blot analysis the amount of 12-lipoxygenase protein in pineal glands was high in the dark and lowest around 16 o'clock. A half life of the enzyme protein was estimated to be approximately 2.8 h in organ culture of rat pineal glands. Northern blot analysis also revealed a higher 12-lipoxygenase mRNA level in pineal glands obtained in the dark than those obtained in the light. Thus, the 12-lipoxygenase of rat pineal glands shows a diurnal fluctuation that is regulated at the transcription level, and may play a certain role in the regulation of neuroendocrine processes of this gland.