Erythropoietin (Epo) production was studied in adult sheep. Nine ewes, body weight (BW) 39 +/- 1.3 kg, were hemorrhaged a volume of blood equivalent to 1.6% BW, and sampled at 0, 2, 4, 6, 24 h. Oxygen content (O2 CT) decreased by 2.7 +/- 0.6 ml/dl at 2 h. Plasma immunoreactive (IR) Epo was only significantly increased at 24 h, from 18.5 +/- 3.5 to 40 +/- 10.7 mU/ml (mean +/- SEM). A further 5 ewes were bled extensively (2793 +/- 82 ml) over 54 h, and killed for Epo mRNA determination. The O2 CT decreased from 12.3 +/- 1.6 to 4.1 +/- 0.6 ml/dl, and plasma Epo increased from 15 +/- 4 to 1675 +/- 287 mU/ml. The sequence of ovine Epo cDNA was derived from the kidney RNA of a severely bled sheep using reverse transcription/polymerase chain reaction (RT/PCR), and from an ovine Epo genomic clone. The cDNA encodes a peptide of 194 amino acids, including a 27 amino acid signal peptide. The deduced amino acid sequence of sheep Epo shows 82%, 78% and 80% homology with mature Epos of human, mouse and monkey, respectively. The gene structure resembles closely those of human and mouse, with 5 exons and 4 introns. The expression of the ovine Epo gene in tissues from normal and hemorrhaged sheep was analysed by a competitive RT/PCR method. Epo mRNA was difficult to detect in liver from normal sheep, but was detectable at 0.01-0.04 amol/microgram total RNA in kidney from normal sheep. In the kidneys of severely bled sheep, the Epo mRNA levels (per micrograms total RNA) increased 400-1500-fold compared to that of normal kidneys, and were approximately 60-fold greater than those in the livers of the hemorrhaged sheep.