To determine the influence of successful or unsuccessful competitive outcomes and small or large margins of victory/defeat on performance, pairs of female undergraduate students "competed" against each other in a dart tossing contest. False feedback about the outcomes and margin of victory/defeat led subjects to believe they were winning every time or losing every time by a margin either consistently large or small. Dependent variables included the number of practice throws made during a period prior to each trial, and point totals for a set of 10 performance throws on each trial. Analysis of covariance was conducted on the data from the postmanipulation trials (trials 2, 3, and 4) and on the data from the premanipulation trial (trial 1) as the covariate. Followup comparisons indicated: (1) The practice behavior of winners remained constant across trials while practice behavior of losers decreased significantly; and (2) When the margin of victory/defeat was small, there was no difference between winners and losers with regard to points scored, but when the difference was large, losers scored more points on trials 2 and 3 but the same number of points on trial 4. It appears that the margin of victory/defeat is a mediating variable in competitive situations. (Author/JD)