In many insects nutrients transferred by the male to the female at mating are later incorporated into both the eggs and soma of the mated females. Accordingly, it has been suggested that female insects can use these male-derived nutrients both for somatic maintenance and to increase both the number and quality of their offspring. Moreover, much discussion is presently devoted to whether the male nuptial gift represents paternal investment, defined as "any increase in given male's total surviving progeny by increasing the reproductive output by a given female", or mating effort which obtains "if a male gains by increasing the proportion of eggs he fertilizes from a given female" (Parker and Simmons 1989). If the male nuptial gift represents parental investment it should be expected to benefit predominantly the offspring sired by the donor, whereas the "physiological fate" of the male nuptial gift is some-what irrelevant under the mating effort explanation. In this paper we test these issues by studying the lifetime fecundity, egg weights and longevity of two groups of females of the polyandrous green-veined white butterfly, Pieris napi, one group of which was allowed to mate only once and the other of which was allowed to mate at liberty, the latter group of females mating on average 2.28 times. Moreover, to test the incorporation rate of male-derived nutrients, we performed a second set of experiments where females were allowed to mate with radioactively labelled males. The results showed that polyandrous females had higher lifetime fecundity compared to monandrous females, laying on average 1.61 as many eggs, and that the difference in cumulative fecundity between the two groups was statistically significant from the 5th day of egg-laying onwards. Polyandrous females also lived longer and maintained egg weight at a high level for longer than monandrous females. Largely concomitant with egg-laying rate, incorporation rate of male-derived nutrients peaked 3-4 days after mating, subsequently tapering off to stabilize at about 40% of the maximum. Given the opportunity, female P. napi remated after 3-5 days, the duration of the refractory period being positively correlated with ejaculate mass. Hence, although the nutrient investment of the first male to mate with a female "subsidizes" the progeny of later-mating males, the male nuptial gift in P. napi clearly qualifies as both paternal investment and mating effort.