Early and late responses to cold (2 °C) and heat treatments ( 40 °C) were analyzed in Allium cepa L. root meristems that were proliferating earlier at 25 °C under steady state kinetics. Under cold treatment, the rates of replication, transcription and translation diminished to roughly a third of their initial values. However, several polypeptides (five neutral ones between 78.1 and 62.4kD, and one acidic of 35.2kD) appeared shortly after the cold, while an acidic 31.3 kD polypeptide was continuously translated after the cold treatment had initiated. Under heat stress, transcription was stimulated and replication decreased, while translation doubled at 2 h and halved at 48 h. The early response was characterized by two neutral polypeptides (of 49.6 and 45.1 kD), while five out of the seven low molecular mass polypeptides (from 24.3 to 15 kD) were only translated at 48h after initiating the heat stress. Three new polypeptides (88.6, 75.5 and 54.6kD) were continuously synthesized after heat. Under both thermal stresses, the same control polypeptide (neutral and larger than 92.5 kD) increased greatly, although only transiently in the cold. From the other seven control polypeptides whose synthesis increased at 2 °C, three of them (of 89.3, 68.1 and 62.4kD) were also over-represented in a transient manner. Finally, at 40 °C, there was a 72.4 kD polypeptide transiently oversynthesized, while two other polypeptides (one neutral of 76.2 kD and other acidic of 70.9 kD) were only overexpressed 48 h after the initiation of the heat stress, the expression of the latter one being strongly reduced 2 h after heat.