Incorporation of rhodamine-labelled serum albumin (R-ALB) into the developing dentine was investigated by fluorescent microscopy. Male Japanese white rabbits were injected with R-ALB at a dose of 50mg/kg and the undecalcified section of the incisor dentine was obtained 11 days after the injection. The orange-red fluorescent labelling of R-ALB was observed in the internal dentine. The labelling consisted of several periodic stripes and decreased its intensity toward the predentine-dentine junction. Serum concentration of R-ALB rapidly decreased after the injection and 10% of the initial value was present after 12 hours. The concentration gradually decreased thereafter. No relationship was observed between the fluorescent labelling pattern and the change in the concentration of serum R-ALB. These results suggest that R-ALB fluorescent labelling is related to the diurnal dentine formation by the odontblasts in the rabbit incisor.