Seasonal changes in natural growth substances were studied by collecting the diffusate, via the pedicel, from intact seeded ‘Winter Nelis’, seeded ‘Bartlett’, and parthenocarpic ‘Bartlett’ pear fruits. The diffusate of ‘Bartlett’ fruits collected 10 to 25 days after full bloom (AFB) contained more auxin-like promoter than did that of ‘Winter Nelis’ fruits. With the exception of the 45-day sample, the diffusate from parthenocarpic ‘Bartlett’ fruits had more promoter from 25 to 70 days (AFB) than did either seeded pear. In contrast, more gibberellin (GA)-like materials diffused from ‘Winter Nelis’ fruits than from seeded or parthenocarpic ‘Bartlett’ fruits. With all types of pears the concentration of abscisic acid-like materials in the diffusate was similar until harvest when the concentration was greater for ‘Winter Nelis’ than for ‘Bartlett’. The combined effect of relatively low amounts of auxin-like and greater amounts of GA-like materials may explain why the presence of seeded pears during the postbloom period has a greater inhibitory effect on flower bud formation in ‘Winter Nelis’ than in ‘Bartlett’.