Tyrosine protein kinases trk, trkB and trkC are essential components of the high affinity receptors necessary to mediate biological effects of the neurotrophins NGF, BDNF, NT-3 and NT-4. Here we report on the expression of these receptors during postnatal development in the rat brain. Cells expressing mRNAs encoding different members of the trk family were identified by in situ hybridization using oligonucleotides complementary to their respective mRNA. In septum, striatum and brainstem, higher levels of trk mRNA were detected at 2 and 4 weeks than at 1 weeks of age. In thalamic nuclei associated with the limbic system, trkB and trkC mRNA were highly expressed at P1 to P7, but the expression declined gradually in 2 and 4 week old animals. Other structures where a developmentally regulated expression was seen included the tenia tecta and piriform cortex where trkB mRNA was not detected until 2 weeks of age. A high labeling was found for trkC mRNA in the deeper parts of neocortex in P1 and P4 animals, while in 2 and 4 weeks old animals the highest labeling was seen over the outer neocortical layers. Several brainstem nuclei showed a higher labeling for trkC mRNA at P1 to P7 than in animals of older age. These data show that expression of members of the trk family is developmentally regulated during postnatal brain development and suggest that high affinity neurotrophin receptors mediate a transient response to neurotrophins in many regions during brain ontogeny.