NGF is essential for the development and maintenance of sympathetic and certain sensory neurons. The NGF receptors on the surface of sympathetic ganglion cells from chick embryos were characterized; they consist of high-affinity receptors with a dissociation constant of about 10(-11) M, and low-affinity receptors with a dissociation constant of about 10(-9) M. There are more than 10 times as many low-affinity as high-affinity receptors per cell. The heterogeneity of NGF binding is not due to negatively cooperative interactions among the receptors. The high- and low-affinity components of NGF binding defined at steady state correspond to slowly and rapidly dissociating components of bound NGF seen in kinetic experiments. In addition, a very slowly dissociating component of bound NGF was observed; this component was a small fraction of binding at low concentrations of NGF but increased to 20-60% of bound NGF at the highest NGF concentrations examined. This very slowly dissociating component of bound NGF accounts for several peculiarities in the binding data not accounted for by steady-state binding of NGF to its high- and low-affinity receptors. Developmental studies showed that both high- and low-affinity NGF receptors were present on chick embryo sympathetic ganglion cells from 6.5 to 20 d in ovo. No significant differences in the numbers or affinities of the receptors were seen with cells from ganglia at 9, 11, or 15 d of development. Cultured non-neuronal cells from sympathetic ganglia had only low-affinity NGF receptors.