Thyrotropin-releasing hormone (TRH) is also present in pancreatic B-cells and its role and regulation here remain unclear. The rat pancreas displays a peculiar ontogenetic pattern for TRH with a rapid increase after birth up to postnatal day 3 when TRH peak is reached. In the present study, dexamethasone (DXM) treatment (1 ¿/g/lOOg BW/day) resulted in an increase of pancreatic weight and retardation of the peak of pancreatic TRH concentration by two days. The TRH-degrading system (either in the 10 000 x g supernatant or in the pellet of pancreatic homogenate) was not stimulated by in vivo DXM treatment. In DXM-treated rats, plasma TSH levels were significantly decreased after postnatal day 1. Plasma glucose concentration was increased on day 1 (i.e. 24 h after the first DXM injection) and decreased to the control level on postnatal day 3. Pancreatic insulin levels were decreased on postnatal day 3 compared to the controls. These results indicate that DXM affects TRH in the neonatal rat pancreas; this effect is probably not mediated through modulation of TRH-degrading activity. The stimulation of pancreatic growth after DXM treatment might be related to the effect on the TRH system.