The rates of synthesis of histone H1 subtypes in synchronized mouse NIH 3T3 fibroblasts were compared with those of rat C6 glioma cells during the G0, G1, and S phases by using a combination of HPLC techniques and conventional gel electrophoresis. In the mouse cell line, all H1 subtypes, H1a-H1e including histone H1(0), were detectable. In the rat cell line, however, no histone H1a was found. H1c and H1e from both cell lines show in the quiescent state a relatively high specific activity comparable with that of H1(0). After release from the G0/G1 block, the synthesis of H1(0) and likewise that of H1c and H1e increase for a short period. All H1 subtypes have their maximum specific activity at the same time after stimulation. The percentage of total H1 specific activity of H1a, H1b, and H1d increases, those of H1c and H1e remain relatively constant, and that of H1(0) decreases while cells cycle from the G0/G1 to the S phase. These findings support our assumption that H1 subtypes could be classified into three groups with common metabolic characteristics: one consists of H1a, H1b, and H1d; another of H1c and H1e; and a third of H1(0) histone. Moreover, the corresponding H1 subtypes from two different species seem to have similar specific activities during the G1 and S phases.