Thyrotropin releasing hormone (TRH) and its precursor peptide pGlu-His-Pro-Gly (TRH-Gly) were measured in serum and in a variety of tissues of developing rats using specific RIA. TRH and TRH-Gly immunoreactivities were detected in most tissues. TRH concentrations were highest in pancreas, in which mean (+/- SEM) TRH concentrations were 138 +/- 20 pmol/g wet tissue 2 d before birth and 644 +/- 80 and 586 +/- 86 pmol/g, respectively, 2 and 5 d after birth. Hypothalamic TRH levels gradually increased from 4 d before birth (12 +/- 2.5 pmol/g) to 77 d of postnatal age (348 +/- 33 pmol/g). Hypothalamic concentrations were lower than levels in pancreas until 13 d of age. The mean serum TRH level at 2 d was 80 +/- 20 pmol/L and fell to the adult range by 21 d. TRH-Gly concentrations were highest in small gut (371 +/- 64 pmol/g) during the neonatal period, falling gradually to adult levels (33 +/- 4.8 pmol/g) by 35 d. Mean hypothalamic TRH-Gly concentrations increased to a peak of 62 +/- 4.5 pmol/g at 13 d, falling thereafter. High TRH-Gly concentrations (greater than 100 pmol/g) also were observed in pancreas (at d 2), kidney, and pituitary gland (at d 21). Serum TRH-Gly concentrations were highest (mean 417 +/- 26 pmol/L) on the 2nd postnatal day and gradually decreased to the adult level by 35 d. Changes in the TRH-Gly/TRH ratio were inversely correlated with tissue TRH concentrations in hypothalamus, pancreas, and liver.(ABSTRACT TRUNCATED AT 250 WORDS)