Neuronal cells from cerebral hemispheres of 14-day-old rats were grown for 6 days in a serum-free, chemically defined medium. About 95-98 and 3% of these cells were neurofilament and acetylcholinesterase (AChE)-positive, respectively. The addition of basic fibroblast growth factor (bFGF) at three developmental stages, i.e. at 4 h, 2 and 4 days resulted in an increase (about 2-fold) of the number of AChE-positive neurons. The enzyme reaction was present in the cell body as well as in the fibers, which often ramified extensively under the influence of bFGF. Treatment with bFGF after the 2nd day of culture had no or only a low stimulatory effect. Our findings indicate that bFGF affects the development of AChE-containing neurons, i.e. cholinoceptive neurons from rat cerebrum.