Abstract 1. 1. In vitro experiments have shown that PGF 2α or indomethacin produces different effects on muscle strips isolated from different compartments of sheep complex stomach. 2. 2. PGF 2α (10 −6–10 −5 M) increases the tone of the ruminal preparations and exerts a negligible effect on the reticulum and omasum strips. 3. 3. Indomethacin (5 × 10 −6 M) inhibits the mechanical activity of the rumen and omasum and has no effect on that of the reticulum. 4. 4. More pronounced are the effects on the different parts of the abomasum: PGF 2α (10 −9–10 −6 M) markedly increases the tone of the proximal abomasum and decreases the amplitude and the phasic contractions of the antral abomasum; indomethacin (5 × 10 −6 M) inhibits the tone and the phasic contractions of the middle abomasum. 5. 5. PGF 2α (10 −9–10 −6 M) inhibits while indomethacin (5 × 10 −6 M) stimulates the contractile activity of the antral abomasum.