Abstract: Effects of different chilling periods were evaluated on growth and development of floral buds of male seedling trees (Pistacia.vera L.) for chilling requirements of male genotypes helpful in predicting overlapping of flowering with female trees and escape from spring cold damage. The chilling requirement and responses of male genotypes to chilling treatment were determined by applying eight levels of chilling to shoots (i.e. 600-1300 h) at 3?1 ^oC. Based on the effect of chilling hours on bud break on four male pistachio genotypes were grouped to early (P₁ and P₆) and late flowering (P₇ and P₁₀) types. Percentage and rate of bud break, duration of flowering, growth and development of bud (length and width) were evaluated. The results indicated that genotypes had different chilling requirement. Among the male pistachio genotypes, the adequate chilling hours (bud break >80%) for P₁, P₆, P₇ and P₁₀ genotypes were 800, 700, 1100, and 1300 hours, respectively. P₁ and P₆ had low chilling requirement (700 hours) for 50% bud break compared to P₇ and P₁₀ (900 and 800 hours). Increased chilling led to decreased heat unit requirements for sprouting, resulting in greater overall growth and development. Chilling was a determining factor in floral bud break for all the genotypes, increasing chilling also produced greater bud break percentages. All genotypes required fewer heat units for bud break as chilling increased. Increasing the chilling hours also increased the length and width of flower buds and reduced duration of flowering.