AbstractLong‐term primary cultures of rat sympathetic neurons require NGF for survival and development. The kinetics of the interaction of125I‐NGF with sympathetic neuron cultures suggests the presence of diffusional barriers preventing a determination of true dissociation and association rate constants. Although the observed rate constants do not accurately reflect the microscopic interaction of NGF with receptor, the ratio of the observed rate constants does provide a good estimate of the KD. This value (1 × 10−9M) agrees with earlier steady state measurements of the KD. The association of125I‐NGF with neuronal cultures is temperature‐dependent with internalization and retrograde transport occurring at 37°C. The retrograde transport of125I‐NGF in compartmentalized neuronal cultures is concentration dependent and saturates at about 100 ng/ml (4 × 10−9M). The amount of125I‐NGF accumulated by retrograde transport appears to be increased subsequent to a period of NGF‐starvation. The increase in uptake does not appear to be due to an increase in NGF receptor number since the number of binding sites is not greatly increased