Summary 1.A comparative study has been made of colonic motility in normal subjects, in patients with ulcerative colitis, and in patients with the irritable colon syndrome. 2.Three open end, water-filled, polythene tubes have been placed in the sigmoid colon and connected to Shillingford-Muller photo-electric transducers which in turn are connected with a direct writing Cambridge three-channel pressure recorder. The response of this system is strictly linear, not liable to electrical drift, and easily calibrated. 3.One hundred and thirty-four separate periods of observation of colonic motility have been made in 110 subjects. There were 23 normal subjects, 46 with the irritable colon syndrome, 36 with ulcerative colitis, and 5 with colostomies. Additional subjects were used in 7 observations of combined cineradiography and pressure recording. 4.The resting pattern of colonic motility over the course of 1 hour varies from subject to subject, but the three clinical groups present different pictures. The following statements are based on a quantitative analysis of the records, beginning with the measurement of amplitude and duration of every wave on all three leads recorded from every subject during the 1-hour period. 5.Records of normal subjects show a preponderance of simple waves of low amplitude and short duration, although bigger and longer waves are also observed. These waves cannot be neatly classified into the types I, II, and III of earlier writers. 6.Patients with the irritable colon syndrome give different resting patterns according to their symptoms. Those with colonic pain as a predominant symptom (spastic colon group) show hyperactive resting patterns while in a stage of symptoms but normal patterns when symptom-free. Patients with pure diarrhea show somewhat reduced colonic activity while in a stage of symptoms but a normal pattern when symptom-free. 7.Patients with ulcerative colitis show hyperactivity when experiencing severe or moderate bloody diarrhea. In a stage of mild symptoms, colonic motility is essentially normal. When symptom-free but with sigmoidoscopic evidence of inflammation, there is reduced motility. When symptom-free and with normal sigmoidoscopic findings, motility is normal.