We studied the effects of TRH on the cytosolic free calcium concentration ([Ca2+]i) of female rat pituitary prolactin-secreting (lactotroph) and GH-secreting (somatotroph) cells in the early postnatal period, i.e. at postnatal days 5 and 10. [Ca2+]i of single identified lactotrophs and somatotrophs was recorded by dual-emission microspectrofluorimetry using the intracellular fluorescent calcium probe indo 1. An application of TRH (100 nM, 10 s) induced a marked [Ca2+]i increase in 65% of neonatal lactotrophs and 34% of neonatal somatotrophs while the remaining cells were unaffected. Most of the responsive cells, both lactotrophs and somatotrophs, exhibited a similar biphasic Ca2+ response, made up of an initial rapid large increase in [Ca2+]i followed by sustained [Ca2+]i fluctuations. In both cell types, removal of Ca2+ from the extracellular medium or addition of the Ca2+ channel blocker, cadmium chloride (500 microM), inhibited the second phase whereas the first phase persisted. Furthermore, in both cell types, protein kinase C (PKC) depletion by incubation in phorbol myristate acetate (1 microM) for 24 h abolished the second phase but did not inhibit the first phase. Conversely, when cells were pretreated with the Ca(2+)-ATPase inhibitor, thapsigargin (100 nM), all TRH-induced [Ca2+]i changes in both cell types disappeared. TRH therefore induces a biphasic increase in [Ca2+]i involving intra- and extracellular Ca2+ in neonatal lactotrophs and somatotrophs as it does in adult lactotrophs. The first phase is presumably due to mobilization of Ca2+ from intracellular stores whereas the second phase presumably results from a PKC-sensitive influx of Ca2+. TRH action on membrane potential was then investigated using the patch-clamp technique in the whole-cell mode. TRH-induced changes in membrane potential consisted of an initial hyperpolarization followed by depolarization and action potential firing. We also investigated TRH action on prolactin and GH secretion by neonatal pituitary cells using RIA. Surprisingly, static assays of prolactin and GH revealed only stimulation of prolactin release by TRH but no effect on GH secretion, although, as expected, GH-releasing factor was a potent agonist of GH secretion. Our results suggest that TRH regulates neonatal lactotrophs and somatotrophs differently, in that the [Ca2+]i changes do not correlate with stimulation of exocytosis in the latter cell type.