The effect of intra-abdominal pressure was investigated in the rat, in which the testes normally descend on or about the 21st day of age. In newborn rats less than 5 days old, the entire anterior abdominal wall musculature was excised from the level of the umbilicus to the xiphoid process. The proximal attachment of the gubernaculum also was incised bilaterally. The abdomen was re-explored on the 28th day to examine the position of the testes. Ninety-five male animals survived. The 4 testes held by adhesions and the 12 with testicular atrophy were excluded from the analysis. A combination of abdominal muscle excision and transection of the proximal gubernaculum (A) reduced the number of descended testes to 19 of 45 (42%). Transection of the gubernaculum alone (B) resulted in 30 of 43 (70%) descended testes. Where an abdominal defect was created without cutting the gubernaculum (C), 34 of 36 (96%) testes descended, which is similar to sham-operated (D) controls 50 of 50 (100%). There was a significant difference between groups A and B (p < 0.025), A and C (p < 0.005) and B and C (p < 0.025). These results show that the gubernacular attachment to the testis and the intra-abdominal pressure act synergistically to aid testicular descent in the rat.