The normal mucosal surface morphology in the rectum has been described by investigation of 15 rectal biopsies using a semimacroscopic technique. The structures of the mucosal surface were rendered visible by staining the biopsies as whole mounts with Alcian Green. A regular pattern of the crypt openings was found, and the goblet cells were observed in and just around the crypt openings. The density of crypt openings was calculated as the number of crypt openings per square millimetre (104 ± 23 openings per mm², mean and S.D.). Histologic examinations of the same biopsies were performed, and the density of crypt tubules was calculated from serial, horizontal sections (130 ± 34 tubules per mm², mean and S.D.). The ratio of crypt openings/crypt tubules was 1:1.25, indicating some branching of the mucosal glands in the rectum.