Electrolytes, metabolites, cortisol and reproductive hormones were measured in maternal plasma taken at least twice daily from three cases of bovine hydrops before, during and after parturition induced by dexamethasone or prostaglandin. Caesarean operations were required for two of the cases. Maternal plasma electrolytes remained within the normal range, but average potassium and creatinine concentrations were higher (9.2 and 0.68 mmol/litre, respectively) than normal (4.7 and 0.42 mmol/litre) in samples of amniotic fluid obtained at calving. Sodium (100 mmol/litre) and chloride (67 mmol/litre) in allantoic fluid were also higher than normal (53 and 20 mmol/litre, respectively). Conversely, creatinine concentrations were lower than normal in allantoic fluid (2.2 vs 13.8 mmol/litre). Oestradiol concentrations were lower than normal in maternal plasma (ranges: less than 20 to 140 pg/ml vs 30 to 440 pg/ml); maximum prostaglandin F metabolite (PGFM) concentrations were slightly elevated (ranges 1.1 to 2.0 ng/ml vs 0.4 to 0.9 ng/ml). Progesterone and cortisol concentrations remained within the normal range; the latter hormone increased markedly in parallel with raised PGFM concentrations. In two cases, the concentrations of reproductive hormones tended to be lower in the amniotic fluid than in the allantoic fluid. For example, progesterone concentrations were 42.8 and 14.9 ng/ml in the amniotic fluids vs 64.2 and 29.8 ng/ml in the allantoic fluids of the two cows; PGFM concentrations were 27.7 and 4.3 ng/ml vs 34.6 and 5.0 ng/ml, and oestradiol concentrations were 1.5 and 3.5 ng/ml vs 1.1 and 6.4 ng/ml in the two fluids, respectively.