Cyclic mechanical loading sufficient to engender strains of physiologic magnitude applied to recently excised canine cancellous bone cores in vitro increased the release of prostaglandin E (PGE) and prostacyclin (PGI2, measured as its breakdown product 6‐keto‐PGF₁α), during a 15 minute loading period in which PG levels were measured in perfusing medium at 5 minute intervals. Peak production occurred in the 0–5 minute sample. Mean levels preload compared to during load were PGE, 2.66 and 3.67 ng/ml (p < 0.002);​ and 6‐keto‐PGF₁α, 543 and 868 pg/ml (p < 0.007). The elevated levels then declined to preload levels during the loading period. However, the 5–10 minute but not the 10–15 minute samples still contained levels greater than preload values. A second 15 minute period of load, 1 h following the end of the first, produced smaller increases in the levels of release that were statistically significant only for the first 0–5 minute sample during load (preload compared to load mean values, PGE, 1.09–1.66 ng/ml, p < 0.02;​ 6‐keto‐PGF₁α, 401–558 pg/ml, p < 0.04). Immunolocalization revealed PGE and 6‐keto‐PGF₁α in lining cells and 6‐keto‐PGF₁α but not PGE in osteocytes. Addition to the medium of 1 μM PGE₂, approximating the concentration produced by loading, had no significant effect on the specific activity of the extractable RNA fraction labeled with [³H]uridine, whereas 1 μM PGI₂ produced an increase similar to that seen previously with loading. These results suggest that both PGE and PGI₂ may be involved in loading‐related adaptive bone modeling and remodeling. The presence of the PGI₂ breakdown product in osteocytes and the RNA response to PGI₂ suggest that this prostanoid may be an early factor in the cascade of events between strain in the bone matrix and subsequent osteoregulatory responses performed by bone cell populations.