Female reproductive effort can be influenced by the quality of her mate. In some species, females increase their reproductive effort by differentially allocating resources after mating with high–quality males. Examination of female reproductive effort in relation to male quality has implications for estimating the evolvability of traits and for sexual–selection models. Accurate quantification of reproductive investment is not possible in many species. Butterflies are an exception, as most nectar–feeding species emerge with almost intact reproductive resources, and in some species males provide nutrients at mating that enhance female fecundity. By manipulating male donations and using radioactive isotopes, we quantified the effect of variation in nutrient provisioning on female reproductive effort in two butterfly species. In the greenveined white butterfly, Pieris napi, females increased their reproductive effort after receiving large male donations. By contrast, in the speckled wood, Pararge aegeria, where males do not provide nutrients, female reproductive effort was independent of male ejaculate. Increased reproductive effort in Pieris napi resulted from the production of more eggs, rather than from investing more resources per egg. In this species donating ability is heritable; hence females laying more eggs after mating with high–donating males benefit both through higher fecundity and through the production of high–donating sons.