Modulation of HMBA on cAMP-PKA and DAG-PKC signal pathways in the cell cycle of MGc80-3 cells showed that: 1.there were positive and negative regulation between them; 2.signal regulation was different in the cell cycle. G1 phase was the most sensitive one among the four phases, cAMP level and PKA activity were increased by 102% and 348%, respectively, as compared with the control; while DAG content and PKC activity were decreased by 51.45 and 32.3%, respectively, as against the control. G2 phase was the second sensitive phase. Effects of HMBA on the signal regulation in M phase were not found. Changes of signal transduction in S phase were different to others.