Pollen grains from two hybrids, WF9xH55 (W) and K64xK55 (K) were collected and a sample from each was cultured immediately (0 h). The remainder was subdivided and stored at 2, 20, and 35° C. At 3, 6, 12, 24, 36, 48, 72, 96, and 120 h, a sample was cultured. The culture medium contained 15% sucrose, 0.6% bacto-agar, 0.03% calcium nitrate and 0.01% boric acid. Storage at 2° C resulted in a large increase in germination percentage in both W and K reaching a maximum at 24 h and then slowly decreasing with additional storage. No germination was observed at 96 h with W and at 120 h with K. The complete loss of germination occurred during a 24 h period and was very abrupt. At 20° C, a similar but less pronounced pattern was observed. However, after 24 h, aggregates of 50-1 000 pollen grains developed during storage in both W and K. Storage of W at 35° C slightly decreased the germination percentage at 3 h and eliminated it at 6 h. Storage of K at 35° C substantially increased the germination percentage at 3 h with further increases in storage periods resulting in the aggregation of grains. This general pattern of an increase at shorter storage periods followed by a gradual decrease as the storage period was extended, was found for pollen tube length and growth rate. In vitro germination characteristics can be substantially altered by the temperature and length of storage and the response to storage is associated with pollen source.