Following four different surgical procedures in lambs 3–5 weeks old, plasma immunoreactive β-endorphin (β-EP) and cortisol were assayed at 15 min and 24 h as determinants of post-operative stress. A threefold increase in mean plasma β-EP levels occurred 15 min after tail docking, and a maximal eight-to tenfold increase occurred in response to castration and/or mulesing with tail docking. Significant increments in mean plasma cortisol levels followed these surgical procedures with the maximal response 15 min after mulesing plus castration with tail docking. The physiologically active ‘free’ cortisol in plasma represents about 25% of the cortisol, as measured, and the two are highly correlated. At 24 h, β-EP levels in all. treated groups were similar to controls, although a small elevation in cortisol levels was still present in the lambs subjected to mulesing. Ultrafiltration of plasma extracts showed that peak β-EP levels contained about 40% immunoreactivity from low molecular weight species (mol. wt < 10 000). By specific radioimmunoassay and reverse-phase high-performance liquid chromatography this comprised about 75% β-EP1−31, the most potent analgesic endorphin, 10% β-EP1−27, and 15% α-N-acetyl-β-EP. Increased β-EP1−31 levels may modulate post-operative pain in lambs.