Flow cytometric determination of DNA amounts in embryos of fully matured dry tomato (Lycopersicon esculentum Mill.) seeds revealed mainly 2C signals. Apparently, most embryonic cells had arrested the cell cycle activity during maturation at the presynthetic G1 phase of nuclear division. During imbibition of tomato seeds, both in water and in PEG solution, an augmentation in the 4C signal of tissues at the embryonic root tip region occurred. This increase was found to be related to cells entering the synthetic phase of nuclear division leading to a doubling of the chromosomal material. In the root tip cells, 4C/2C ratios rose 1 day after imbibition in water while radicle emergence started 2 days later. Apparently, DNA synthesis preceded germination. The induction of nuclear activities was compared with changes in protein expression patterns during seed imbibition. Using chemiluminescent immunodetection after Western blotting, we identified changes in /3-tubulin expression during germination and priming of tomato seeds. Tubulin was found to be synthesized 8 h after the start of imbibition both in water and in PEG. The start of tubulin synthesis considerably preceded DNA replication, cell division and germination. The induction of cell cycle activities may be an early indicator of the initiation of cell division and the start of germination.