Plasma progestagen concentrations were measured daily by radioimmunoassay (RIA) in 35 sick foals for the duration of their illness. The foals were divided into three groups on the basis of time to stand after birth. Foals were given intensive care treatment according to the severity of their illness. Plasma and urine concentrations of pregnenolone (P5) and pregnenediol (P5 beta beta) were measured by gas chromatography--mass spectrometry; plasma cortisol concentrations were measured by RIA and the foals' renal and respiratory status were assessed by creatinine clearance ratios and arterial oxygen concentrations respectively. Five patterns of plasma progestagen concentrations were identified; in general, values increased when the foal's clinical condition deteriorated and decreased as the foal improved. Median progestagen concentrations decreased over the first three days post partum in Group 1 foals but remained elevated in foals from Groups 2 and 3. Similar changes were observed in plasma P5 and P5 beta beta concentrations. Plasma cortisol concentrations were highest in foals from Groups 2 and 3 (P < 0.01) compared with foals from Group 1. Regardless of foal group, mean cortisol concentrations were highest (P < 0.001) in those foals treated with adrenocorticotrophic hormone compared with those treated with dexamethasone or with neither drug. There was no relationship (r2 = 0.21) between plasma cortisol and progestagen concentrations. Results from renal clearance, steroid conjugation and respiratory status suggest that these factors did not play a significant role in elevating progestagen concentrations in sick foals. It is hypothesized that there may be a relationship between adrenal stimulation and an enzyme block resulting in overproduction of P5 and P5 beta beta in the sick neonatal foal.