The effects of growth promoting activity (GPA) on sympathetic neurone development were investigated in vitro and compared with the effects of ciliary neurotrophic factor (CNTF). GPA interfered with sympathetic neurone proliferation and induced the expression of vasoactive intestinal peptide (VIP) in neurones from 7-day-old (E7) chick embryos. The biological effects observed with saturating levels of GPA are indistinguishable from the effects of CNTF. The effects on VIP expression suggest that GPA may be involved in the specification of sympathetic neurone phenotypes. Whereas half maximal effects are achieved at lower concentrations of GPA than CNTF, GPA competes less efficiently than CNTF for the binding of 125I-labelled CNTF. This suggests similar, but not identical interactions of CNTF and GPA with receptors on chick sympathetic neurones.