To investigate the biological role of basic fibroblast growth factor (bFGF) for the development of the spinal cord we studied the in vitro and in vivo effects of this protein on survival and choline acetyltransferase (ChAT)-activity of embryonic chick and rat spinal cord neurons. In vitro, bFGF (ED50 1-2.8 ng/ml) supported the survival of embryonic neurons from the ventral part of the rat spinal cord (ventral spinal cord, vsc), including motoneurons. Addition of bFGF (100 ng/ml) increased the ChAT-activity in embryonic chick vsc cultures to 150% as compared to untreated cultures (100%). The effect of bFGF was dose-dependent. In vivo-application of bFGF resulted in a similar increase of ChAT-activity in chick spinal cord. Since bFGF stimulates the ChAT-activity of spinal cord neurons in vivo and in vitro we therefore conclude that this protein may have a physiological function for the transmitter development of cholinergic spinal cord neurons.