Developmental changes in pituitary glucocorticoid receptor (GR) mRNA were examined during gestation and early neonatal life using in situ hybridization. Pituitaries were harvested from sheep fetuses at days 60-80, 100-120, 130-135, 140-142 and term, and from lambs of days 0-7 and 30-60, and adults. GR mRNA was present in the pars distalis by day 60, levels increased through gestation, and there was a redistribution of GR mRNA, resulting in a relatively greater abundance at the base of the pars distalis. At term, there was a significant (P < 0.05 compared with the day 140-142 fetuses) elevation of GR mRNA, which was maintained in the newborn lamb, reaching highest levels at days 30-60 of neonatal life. GR mRNA was undetectable in the pars intermedia until day 120, but subsequently increased to high levels at term. Interestingly, the expression of GR mRNA in the pars intermedia dropped precipitously in the newborn (P < 0.05 compared with term), though levels recovered in the older lambs and adults. The regional and cellular distribution of GR mRNA correlated closely with the presence of immunoreactive GR (irGR) in the pituitary; the majority of irGR was present in the nuclei. Intrafetal infusion of cortisol (12 h; 5 micrograms/min) in late gestation (day 135) had no effect on Gr mRNA expression in either the pars distalis or pars intermedia.(ABSTRACT TRUNCATED AT 250 WORDS)