1. Basal and haemorrhage-stimulated erythropoietin (Epo) and ACTH levels were measured in the chronically cannulated immature ovine foetus (less than 125 days) by radio immunoassay (RIA). 2. Basal erythropoietin levels were found to be higher than those previously reported in the late gestation (greater than 130 days) ovine foetus, but were lower than those observed in the neonatal lamb. 3. In control foetuses (Protocol 1) the small degree of haemorrhage associated with the sampling procedure increased the plasma Epo values from 11.4 +/- 3.0 (n = 5) mU/mL to 23.8 +/- 4.3 mU/mL at 24 h (mean +/- s.e.m.). There was a significant monotonic increase with time (F = 16.4; d.f. 1,19; P = 0.001). An initial haemorrhage of approximately 10% blood volume (Protocol 2) increased plasma Epo values from 7.3 +/- 2.3 to 24.2 +/- 7.1 mU/mL (n = 3). 4. Haemorrhage of 20% fetal blood volume (Protocol 3) produced an increase in plasma Epo from 9.3 +/- 1.7 to 54.7 +/- 15.5 mU/mL at 6 h and to 57.6 +/- 7.3 mU/mL at 24 h (n = 5). By repeated measures ANOVA, the effect of the 20% haemorrhage was significant when compared with the control group (F = 7.32, d.f. 2,16, P = 0.006). There was a significantly greater decrease in haematocrit (F = 6.7, d.f. 2,20, P = 0.004) and haemoglobin (F = 5.0, d.f. 2,20, P = 0.013) in animals of Protocol 3 than in those of Protocol 1. 5. Fetal blood gases and plasma adrenocorticotropic hormone (ACTH) did not alter with haemorrhage, indicating the tolerance of the foetus to this degree of haemorrhage.