It has been assumed that in piebald lethal mice that develop megacolon, impaired colonic motor activity is restricted to the aganglionic distal colon. Peristaltic mechanical recordings, immunohistochemistry and quantitative PCR were used to investigate whether regions of the colon, other than the aganglionic segment, may also show anatomical modifications and dysfunctional colonic motor activity. Contrary to expectation, colonic migrating motor complexes were absent along the whole colon of piebald lethal homozygote mice; and severely impaired in heterozygote siblings. Aganglionosis was detected not only in the distal colon of piebald homozygote lethal mice (mean length: 20.4 ± 2.1mm), but also surprisingly, in their heterozygote siblings (mean length: 12.4 ±1.1mm). Unlike homozygote lethal mice, piebald heterozygotes showed no signs of megacolon. Interestingly, mRNA expression for PGP 9.5 was also dramatically reduced (by 71-99%) throughout the entire small and large bowel, in both homozygote lethal and heterozygous littermates (by 67-87%). Histochemical staining confirmed a significant reduction in myenteric ganglia along the whole colon. In summary, the piebald mutation in homozgote lethal and heterozygote siblings is associated with dramatic reductions in myenteric ganglia throughout the entire colon, and not limited to the distal colon, as originally thought. Functionally, this results in an absence, or severe impairment of colonic MMC activity in both piebald homozgyote lethal and heterozygote siblings, respectively. The observation that piebald heterozygotes have an aganglionic distal colon (mean length: 12mm), but live a normal murine life span without megacolon, suggests that aganglionosis >12mm and the complete absence of colonic MMCs may be required before any symptoms of megacolon arise.