Two polyethylene melts have been examined in elongational flow using two different experimental techniques: the rod‐stretching method of Münstedt and co‐workers, and the bubble‐collapse method of Middleman and co‐workers. Good agreement is observed in the case of a high density polyethylene. For the low‐density polyethylene the bubble‐collapse method appears to give incorrect results. It seems likely that for this highly elastic melt the time scale of bubble collapse is too short to permit attainment of a steady stress, even though the kinematics do reach constant stretch rate.