To estimate the absorbed dose of N-ethyl-N-nitrosourea (ENU) ingested in Drosophila melanogaster, males were fed with sucrose solutions containing various concentrations of ENU plus 3H-labeled sucrose for 24 hr. Flies showed decreasing intakes with increase in ENU concentration when monitored by intake 3H radioactivity. Absorbed dose, D, per male can be estimated by the following formula: D = v . C, where v is intake volume per male of sucrose solution at concentration C. Estimating the v value for each ENU solution from 3H radioactivities in male flies, and using the above formula, we reached the conclusion that average absorbed doses of ENU were 0.064, 0.221, and 0.302 nmol, respectively, for the ENU concentrations of 0.03, 0.3, and 1.0 mM. Sex-linked recessive lethals were measured for males exposed to these sucrose solutions at three different ENU concentrations. Their frequencies increased nonlinearly with increasing exposure doses--ie, ENU concentration C--but linearly with increasing absorbed doses estimated in the above-mentioned way.