The isolated strips of tortoise intestine are evaluated as a test organ for bioassay of prostaglandin E₁. This preparation responded highly sensitively to PGE₁ and PGE₂ in picogram concentration range. The mean slope and the value of precision index among the doses of 0.1, 0.3 and 0.5ng/ml in final concentration were 37.7 and 0.143, respectively. And this was relatively insensitive to different prostaglandins; E₁/E₂ ≳ 1, E₁/A₂ ~ 50 and E₁ /F_2^α ~ 100, and showed the dual responses to 5-hydroxytryptamine and histamine; initial contraction followed by relaxation. The dose-ratio inducing the relative equal contraction height for PGE₁, acetylcholine, caerulein, angiotensin and barium chloride was 0.4 : 50 : 25 : 10 : 100 in this order. These results suggest that the intestinal strips of the tortoise are suitable for bioassay of prostaglandin E₁ and E₂ between the doses of 0.1 and 1.0 ng/ml level in the tissue extracts.