Abstract— The synthesis of brain chromosomal proteins was investigated after a pulse of the nervous system‐specific carcinogen N‐ethyl‐N‐nitrosourea to the fetal BD IX‐rat. At different times up to 34 days after the carcinogen pulse, carcinogen‐treated and control brain cells were incubated for 6h with [14C] and [3H]leucine, respectively, in a standardized suspension culture system. The relative rates of leucine incorporation into histones F2a1, F2a2. and F3, and into 13 individual non‐histone chromosomal proteins, were measured after electrophoretic separation in 15% and 10% polyacrylamide gel, respectively. Both histone and NHP synthesis showed a biphasic response, with a common 2nd maximum as late as 15 days after the carcinogen pulse. A first peak of non‐histone chromosomal protein synthesis was seen at 24 h, when histone synthesis had not yet recovered from an initial 48 h depression. The data are discussed in relation to the possible regulatory role of chromosomal proteins in gene transcription during neoplastic transformation.