The external and internal oblique, transverse abdominis and the rectus abdominis muscles constitute the anterolateral abdominal wall muscles. They are striated and contract voluntarily. We investigated the hypothesis that contraction of these muscles by coughing or straining, can also occur as a reflex. Effect of straining on muscles was tested in 19 healthy volunteers. The intra-abdominal pressure was measured by a manometric catheter introduced into the rectum. The response of the muscles to straining was recorded by a needle electrode inserted into each of the muscles. Similar to voluntary contractions, sudden and sustained straining produced increase in the rectal pressure and the motor unit action potentials of each of the muscles which was abolished by anesthesia. These findings suggest presence of a 'straining-abdominal wall reflex'.