The prenatal and postnatal expression of calretinin was studied in hippocampus of the rat using immunohistochemical procedures. Calretinin was detected as early as embryonic day 15 in the primordial hippocampus where calretinin-containing neurons and fibres were localized to the primitive plexiform layer. Upon emergence of the hippocampal plate (the prospective stratum pyramidale), large numbers of immunopositive multipolar cells were observed in the marginal zone. Fewer cells with fusiform cell bodies were observed bordering the hippocampal plate and subplate. During the perinatal period (embryonic day 20 to postnatal day 0), large numbers of immunoreactive pyramidal-like neurons were observed at the margin of the hippocampal plate with the subplate. At this same time, many calretinin-containing neurons with irregularly shaped dendrites were observed in stratum radiatum. Soon after birth (postnatal day 3), the calretinin immunoreactivity of both these later cell types rapidly declined and a new population of calretinin-immunopositive cells emerged, the Cajal-Retzius cells of stratum lacunosum-moleculare and the dentate gyrus. The Cajal-Retzius cells rapidly matured but disappeared by the second postnatal week. During the second postnatal week, calretinin interneurons of the adult hippocampal formation began to appear. Their immunoreactivity increased by postnatal day 15, when the number of calretinin-immunopositive interneurons in area CA1 and stratum radiatum of CA3 exceeded that of the adult. At this time, the soma and proximal dendrites of many calretinin interneurons were found to contact each other. The frequency of such cellular appositions decreased in adulthood. The results presented here show that calretinin immunohistochemistry can be very useful in recording the development of subpopulations of hippocampal neurons that are present during distinct embryonic and postnatal periods. Although some neuronal types may exist only briefly during hippocampal development, others appear to express calretinin transiently during restricted phases of neuronal differentiation. Surprisingly, this includes some hippocampal pyramidal cells. However, even as the adult pattern of immunostaining emerges in week 2, morphological refinement of interneurons continues to take place, which eventually leads to the population of calretinin-containing interneurons of the mature hippocampus.