When maintained in some serum-containing media, fetal rat sympathetic neurons acquired substantial choline acetyltransferase activity and form cholinergic synapses in vitro. However, when they are maintained in a serum-free, defined culture medium, choline acetyltransferase activity is not detected and cholinergic synapses are not observed. In this study, we have examined the effects of various times of exposure to a medium inducing cholinergic function on the properties of neurons subsequently maintained in defined medium. We report that 2-day, but not 2-h, exposure to this inducing medium causes a long-lasting (>6 weeks) increased (7-10-fold) in the activity of choline acetyltransferase and that, under these conditions, sympathetic neurons in vitro form cholinergic, electrical and mixed function cholinergic and electrical synapses. We conclude that a relatively brief exposure to media inducing cholinergic function can cause long-lasting changes in the functional properties of sympathetic neurons in vitro.