Acetyl- and succinylisocupressic acids were prepared by derivatization of isocupressic acid isolated from ponderosa pine. The isocupressic acid derivatives were tested and found to induce abortions in late gestation cows after oral dosage starting on day 250 of gestation. Incubation of acetyl- and succinylisocupressic acid in bovine ruminal fluid resulted in hydrolysis of the two esters and the production of isocupressic acid. The abortifacient activity of the isocupressic acid derivatives was thus attributed to their ruminal conversion and production of isocupressic acid. Keywords: Pinus ponderosa; premature partuition; bovine abortion; ruminal ester hydrolysis