Glucocorticoid feedback plays a major role in regulating HPA axis activity and this mechanism occurs via two different receptors: mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs). Glucocorticoid receptors are thought to mediate negative feedback signals of elevated glucocorticoid levels, present at the circadian peak or following stressful stimuli. In this study we investigated the effect of 5 days treatment with the GR antagonist ORG 34850 [1] on basal and stress-induced corticosterone release in rats. Male rats were anaesthetized with a combination of Diazepam (0.2 ml, I.P.) and Hypnorm (0.2 ml, I.M.), and a catheter was implanted in the right jugular vein for blood samples; during the same surgery a subcutaneous cannula for drug administration was inserted. Treatment with ORG 34850 (10mg/kg, S.C., 2/day; n=9) or vehicle (5% mulgofen in saline, 1ml/kg, S.C., 2/day; n=7) started after recovery from anesthesia and was carried on for 6 days. On the 5th day of treatment, an automated blood sampling system (ABS) collected blood from each freely moving, unanaesthetized rat every 10 min over a 24-h period; in a separate set of animals (n=4 for each group), on the 5th day rats were exposed to a white noise stress for 10 min (114 db) and blood samples were collected using the ABS; on the 6th day, the same animals were restrained for 60 min and blood samples were collect by hand. At the end of experiments animals were killed and samples were analysed for corticosterone content using radioimmunoassay. Statistical analysis of 12 hour sampling period (2pm-2am) using Student’s t test revealed that in rats treated for 5 days with ORG 34850 compared with controls showed an increase on total basal corticosterone release (684±46 ng compared with 504±42 ng; P= 0.017), on mean corticosterone concentration (57±4 ng/ml compared with 42±4; P= 0.014) and on height of corticosterone pulses (100±17 ng/ml compared with 78±6 ng/ml; P=0.031). This effect was marked during the rise phase of corticosterone, from the early afternoon until the early morning. The same treatment had no effect on HPA axis response to noise or restraint stress: statistical analysis using repeated measure test showed no significant effect of the treatment with ORG 34850 in rats exposed to noise or restraint stress, compared with the control group (peak corticosterone levels were 174.5± 16.5 compared with 188.3 ±14 ng/ml for rates exposed to noise and 397.4±47 compared with 459.8±52 ng/ml in rats exposed to restrain). Our data show that prolonged blockade of glucocorticoid receptors induced an increase on basal but not stress-induced corticosterone release. Those changes in the HPA axis activity could be related to effects on GR-mediated negative feedback. Moreover, our data show that the GR-mediated negative feedback may be acting through a different mechanism on the modulation of basal or stress-activated HPA axis activity.