Flow cytometry was used for comparative in vivo and in vitro analysis of cell populations staining positively for somatostatin. Experiments were carried out with pineals obtained from neonatal, 8- and 15-day-old rats. Pineal cells were obtained by dispersion with collagenase and then processed in a flow cytometer or maintained in culture for 1 or 2 weeks. Identification of somatostatin-immunopositive cell populations was performed using a polyclonal somatostatin antibody and confirmed by indirect immunostaining of cytospun smears with the avidin-biotin-peroxidase method. In vivo, the percentage of somatostatin-positive cells was 60.6 +/- 4% in neonatal pineals and declined to 22.2 +/- 11% in 15-day-old animals (p < 0.04). The density of peptide immunostaining decreased in 8-day-old animals but recovered to the neonate levels in 15 day-old animals; homogeneity in the immunopositive population increased with age. Maintenance in culture for 1 week resulted in an increase in positive somatostatin staining in animals of 8 and 15 days with no changes in neonates; however, after 2 weeks of culture, the percent of immunopositive cells decreased from 53.3 +/- 6 to 12.2 +/- 4% in the older animals and remained unchanged in neonates. We conclude that somatostatin is found in pinealocytes and shows a declining pattern during the perinatal period; this probably implies that the peptide plays a paracrine role important for cell differentiation in these young animals, since maximal cellularity and a high mitotic index occur within the first 3 days of life, and pineal cell differentiation is completed before the end of the third week of extrauterine life.