Different concentrations of urea sprays were applied to chemically thin shoots and whole trees of peach and nectarine (Prunus persica (L.) Batch, cvs Maravilha, Sunred, Rhodes and Hermosa) at the closed pink-bud stage, at full bloom (FB), and at the early fruitlet stage. The effect of thinning was evaluated by recording fruit set periodically on preselected shoots, weighing and counting the number of fruitlets removed when hand-thinning correction was carried out, and by weighing mature fruit at harvest. Urea significantly reduced fruit set, increased fruitlet weight as early as one to two weeks after application, and increased the final fruit weight of all cultivars tested. Yield, calculated on the basis of unit trunk cross-sectional area, was not reduced, except in ‘Sunred’ thinned with 16% urea. The open flower stage was more sensitive to urea than the closed pink bud or the fruitlet stage in ‘Rhodes’ and ‘Hermosa’, but not in ‘Maravilha’. Although sensitivity of the reproductive organ to urea varied, thinning could be accomplished at all three stages. The higher concentrations of urea (12% to 16%) caused some dieback of small twigs after penetration through unopened resting buds.