A population balance equation is solved numerically to obtain the transient size distributions of high-viscosity drops dispersed in agitated vessels. Three parameters included in the equation, such as breakup frequency, daughter drop distribution and the number of daughter drops formed per breakup of a parent drop, are proposed based on the author＇s previous studies for high-viscosity drops (Kuriyama et al., 1995 a, 1995 b, 1996) and the previously proposed model for the breakup of low-viscosity drops (Konno et al., 1983). Measurements of transient drop-size distribution were made under various operating conditions. Through comparison of calculated distributions with experimental ones, the drop-breakup model proposed in this work is found to be capable of predicting the transient drop-size distributions over a wide range of drop viscosities.