The dose/response relationship of the kinetostatic effect of demecolcin (ColcemidR) and vinblastine sulphate (VelbeR) was estimated in the epidermis, the forestomach, the jejunum and the colon of the hairless mouse (hr/hr) at 3 hours after intraperitoneal (i.p.) injection. A dose of 0.15 mg Colcemid per animal was found to be the lowest effective dose in all the organs tested. The value did not differ much from the corresponding effective dose of Velbe. This dose of Colcemid was then used to study the accumulation of arrested mitoses during the first 4 hours after i.p. injection. An almost linear increase in the mitotic count was found up to 4 hours in the epidermis, 3.5 hours in the forestomach and 3 hours in the jejunum and the colon. A dose of 0.15 mg Colcemid per animal i.p. with an observation time of 3 hours is thus a reliable method for estimation of the mitotic rate in many organs. It is also concluded that both Colcemid and Velbe can be used as reliable stathmokinetic drugs for both epidermis, forestomach and jejunum and colon mucosa provided that correct doses and short enough accumulation time (3 hours) is used.