4 female Wistar rats,weighing 287.5?21.7 gms,about 7～7.5 months old,were used.Using ~3H-TdR and ~(14)C-TdR as labels, double labelling method was employed. The dose of ~3H-TdR used was 1 ?Ci/gm body weight and that of ~(14)C-TdR used was 0.125 ?Ci/gm body weight. Bone marrow smears of both femurs were made and autoradiographs were coated with No. 4 liquid nuclear emulsion.The following results were obtained:In erythropoietic series: Nc/N_H=5.0?1.0, LI=62.3?17.9％, MI=3.4?1.9％. According to the above figures, by calculation, we obtained the cell cycle time and the duration of 4 phases as follows: TG_1≤4.3 hrs,Ts=5.0 hrs, TG_2≥1.0 hr, Tm=0.4 hr and Tc=10.7 hrs.In neutrophilic granulopoietic series: Nc/N_H=3.9?0.9, LI=45.3?5.3％, MI=3.3?1.4％. According to the above figures, the following parameters of the cycle. were obtained as follows: TG_1≤5.6 hrs, Ts=3.9 hrs, TG_2≥1.0 hr, Tm=0.4 hr and Tc=10.9 hrs.There was no obvious difference of Tc between the erythropoietic series, and theneutrophilic granulopoietic series in rats, but significant difference of Tc between them in mice.