We examined the androgenic regulation of protein kinase CK1 in rat ventral prostate cytosolic and chromatin fractions. On androgen deprivation, CK1 in both the nuclear and cytoplasmic compartments decreases slowly at a similar rate. Stimulation of regrowth in the prostate does not evoke an early differential modulation in the CK1 because it increased in both the cytosol and chromatin only by 24 h after androgenic stimulus. These changes in CK1 relate to its proposed role in cell regulation at mitosis and differ from those in protein kinase CK2 that demonstrates rapid modulations in the nuclear compartment under similar conditions.