In Physarum polycephalum the triggering of histone H4 gene transcription occurs in G2 phase. The rate of synthesis of histone H4 mRNA was measured by in vivo pulse-labeling experiments. We show that it begins to increase in mid-G2. During the second part of G2 it increases approximately 20 fold over its minimum value and reaches a maximum at the end of G2. After entry of the cells in S, histone H4 gene transcription rate begins to decrease and reaches a minimum value in early G2. The histone H4 mRNA which accumulates in G2 is not translated immediately into proteins but is stored in an inactive form until the beginning of the next S phase. Immediately after its transcription the H4 mRNA is transported to the cytoplasm where it is stored and stabilized as an inactive mRNP complex. This was shown by fractionation of cytoplasmic RNP in sucrose gradients and blot hybridization of subcellular fractions. © 1988 Academic Press, Inc.