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Hollow heart, a pea seed disorder, reduces pea seed vigour. It is associated with high temperature after seed set, but the susceptible seed development stage is not entirely clear. A field trial in New Zealand involving three sowing times (September, October, and November 2003) and three garden pea (Pisum sativum) cultivars ('Rainier', 'Early Onward', and 'Alderman') was conducted to determine if the predisposing stage to the disorder could be identified. Hourly thermal time (HTT) at a base temperature (Tb) of 25°C when seed moisture content (SMC) was 70-80% was correlated positively with hollow heart incidence at harvest maturity (HM), but this response differed among cultivars. HTT (Tb = 25°C) when SMC was 15-25% was not correlated with hollow heart incidence at HM, if data for all cultivars were included in the correlation analysis. However, an increase in hollow heart incidence after seed had dried to between 15% and 25% SMC occurred with increasing HTT (Tb = 25°C) in 'Alderman'. These results demonstrate that there are genetic differences in susceptibility to hollow heart. The hypothesis that hollow heart is associated with a starch deficiency in the adaxial region of the cotyledons, and that cultivar differences are related to differing efficiencies of assimilate transport into seeds at high temperatures is discussed. [ABSTRACT FROM AUTHOR]