Using the specific affinity of tubulin for colchincine and the strong absorption of tubulin to DEAE ion exchangers at neutral pH and moderate ionic strength, the tubulin amounts in the brain from both mice and chicks in different developing stage was quantitated by ³H-colochicine assay (expressed as colchicine binding activity). The results showed that the rate of tubulin synthesis was a peak value period in the brain developing critical period This is exactly a period in which thyroid organization and function are being perfected. Besides, in breeding period, the difference of tubulin content between male and female is significan (P<0.001). The tubulin synthesis is strictly sex dependent (only brain from sex maturity stage). We suggested that the sexual hormone might exert their effect in tubulin synthesis.