Manual crushing of one blastocyst performed on 181 bicornuate twin pregnancies between Days 24 and 45 has shown that the uncrushed blastocyst can either survive and develop normally to full term or may be rejected and resorbed like its crushed twin, depending mainly on the stage of pregnancy. Crushing, which causes rupture of fetal membranes, results in a rapid fall in the survival rate of the uncrushed blastocyst when performed after Day 31. In some mares rupture is not possible after Day 35 even if extreme pressure is used. Crushed, but unruptured, blastocysts mainly between Days 35 and 45 may (a) resorb as does the uncrushed twin (60%) (b) resorb without loss of the uncrushed twin (20%) or (c) survive, as does the uncrushed twin, even when several attempts on alternate days are made to destroy the blastocyst (20%). At no stage of pregnancy is the uncrushed embryo resorbed unless the crushed twin is destroyed. The rejection and resorption of the uncrushed twin is not due to transference of pressure, but presumably to physiological processes involving prostaglandin. Mefenamic acid appears to increase survival rates especially when crushing is performed after 31 days, although the twin of apparently successful crushings at this stage may be lost between Days 50 and 70.