Hard gelatin capsules, containing riboflavin-loaded poly (N-vinyl-2-pyrrolidone)-polyacrylamide cylindrical hydrogels, were modified chemically by treating with an aqueous formaldehyde solution for the purpose of delayed release of drug along the gastrointestinal tract. The t(dis) (disintegration time) of capsules was studied as a function of concentration of formaldehyde solution and the treatment time. The dynamic release of vitamin B-2 was studied as a function of crosslinking ratio of the hydrogels. The device studied seems to have potential to be used for colon-targeted drug delivery. (C) 2003 Wiley Periodicals, Inc.