Primary cultures of rat hippocampal cells have been used to evaluate trophic effects of neurotrophin-3, brain-derived neurotrophic factor, nerve growth factor, and basic fibroblast growth factor. There was little survival in cultures prepared from embryonic day 17 embryos and grown in defined medium without growth factors. Addition of basic fibroblast growth factor produced a massive increase in the number of neurons present in the cultures seven days after plating. This action reflected proliferation of neuronal precursor cells rather than increased survival of initially plated neurons. Brain-derived neurotrophic factor was ineffective under these conditions, whereas neurotrophin-3 produced a very small, but statistically significant increase in neuronal survival in the range of 20%. However, hippocampal neurons were responsive to brain-derived neurotrophic factor and neurotrophin-3 as demonstrated under culture conditions, resulting in survival in absence of the neurotrophins. Acute administration of brain-derived neurotrophic factor and neurotrophin-3 to hippocampal cultures grown at high density stimulated the hydrolysis of phosphatidylinositol, a response earlier shown to be mediated by tyrosine receptor kinase neurotrophin receptors. Furthermore, when such cultures were grown in presence of neurotrophin-3 rates of glutamate and GABA uptake were increased. In contrast to the findings obtained in cultures of embryonic day 17, cultures prepared from embryonic day 14 or 15 animals were viable in absence of exogenous growth factors. The specific neurotrophin receptor inhibitor, K-252b reduced survival in these cultures and this effect was partly overcome by exogenous neurotrophin-3. Our findings suggest that hippocampal neuron survival at early embryonic stages may involve paracrine neurotrophin mechanisms, whereas the survival of hippocampal neurons of embryonic day 17 is not markedly enhanced by brain-derived neurotrophic factor or neurotrophin-3. However, at this embryonic stage there is a functional response to both neurotrophins as made evident by the activation of tyrosine kinase receptor-linked signal transduction mechanisms and by the stimulation of transmitter-specific differentiation.