The program of Hl histone synthesis in S. purpuratus embryos and the control of its timing. The program of Hl histone synthesis was investigated in Strongylocentrotus purpuratus embryos in order to determine when the late Hl histones are synthesized. The embryos were staged by cell number, morphological stage, and chronological age. Care was taken to ensure that conditions of rearing the embryos remained consistent. Labelling with 3H-lysine revealed that two forms of early Hl (Hl alpha 1 and Hl alpha 2) are synthesized from early cleavage stages (16-32 cells) to the blastula stage. Hl alpha 1, synthesis ceases first at around 400 cells (hatching blastula); Hl alpha 2 continues to be synthesized by the blastula but is usually absent from the labelling pattern in early gastrula (700 cells). Two forms of late Hl appear sequentially during the pre-hatching blastula stage. Hl gamma appears at 200-250 cells and Hl beta approximately 1 hr later, at 250-300 cells. The late forms of H2A and H2B are first synthesized at around hatching, several hours after late Hl forms appear. The time of synthesis of late Hl is strictly controlled by a chronological clock and is not affected by disruption of the cell cycle, cessation of DNA synthesis, or alteration of nucleocytoplasmic ratios. Polyspermic embryos and hydroxyurea-arrested embryos followed the normal temporal program of onset of late Hl synthesis.