Four species of herbaceous plants (Sanguinaria canadensis L., Jeffersonia diphylla (L.) Pers., Erythronium americanum Ker, and Trillium sessile L.) commonly found in the understory of mature deciduous forests in the eastern United States were subjected to partial defoliation in the spring of 1990 at the time of flowering. Data were gathered on the reproductive responses of control and defoliated plants in 1990 and again in 1991. Partial defoliation had little effect on reproduction in any of the species in 1990, but 50% defoliation significantly reduced multiple fruiting by J. diphylla in 1991, while 67% defoliation significantly diminished three measures of reproductive performance in T. sessile. Defoliation of 50% did not influence reproduction in E. americanum in either year. Experimental defoliation (50%) did not affect reproduction in S. canadensis in 1990, but complete defoliation by deer and ground hogs eliminated reproduction in 1990 and 1991. In these four species the negative consequences of herbivory on reproduction are often delayed, and may not be significant until the level of defoliation reaches at least 50%.