Pollen tubes grow through the stigma and the style after pollination of spermatophyta, whereupon fertilization can take place in the ovules. During the growth of hundreds of pollen tubes through the style, an interaction occurs between the style tissue and the pollen tubes. This interaction finds expression in various kinds of metabolic changes which are coupled with alterations in enzyme activities. Some of these changes in enzyme activities are examined following compatible pollination of Petunia hybrida. The acid phosphatases exhibit a decrease in activity after pollination. The other investigated enzymes show an increase. Following pollination the increase in activity of uridine diphosphate glucose dehydrogenase and the aspartate carbamoyltransferase can be ascribed to the characteristic activities of the pollen tubes. There is a small increase in the alanine aminotransferase, the aspartate aminotransferase (AsAT) and the glutamate dehydrogenase (deamination) (GDH deamin.) that cannot be ascribed to the increase in activity caused by the pollen tubes alone. Large increases in activity of the enzymes of the carbohydrate metabolism take place, viz. of the ketose-l-phosphate aldolase (KPA) and the citrate synthase. An important enzyme for the amino acid synthesis, the glutamate dehydrogenase (amination) (GDH amin.) shows also a great increase in activity after pollination. It has become clear from interaction experiments that an exchange of substances occurs between the pollen tubes and the untouched section of the style where pollen tubes have not yet grown. The GDH amin. and the AsAT show a greater activity in the untouched tissue which anticipates the growth of the pollen tubes. Thus, a wave of enzyme actvities arise in the style preceding the growth of the pollen tubes. The increase in the KPA activity, however, is localized to the site of the pollen tube growth. The uptake into the crude extract and the incorporation into the protein fraction were determined before and after pollination with different 14C-amino acids. Both before and after pollination, there is an uptake of different 14C-labelled amino acids into the crude style extract and an incorporation into the protein fractions. The incorporation into the proteins of the difficult to metabolize amino acids, namely 14C-leucine and I4C-threonine, is 10% greater after pollination, but the uptake in the crude extract is about the same before and after. There is less 14C-proline and 14C-protein hydrolysate taken up and incorporated in the pollinated styles because a portion of these easy to metabolize amino acids is decomposed or is used for pollen tube wall synthesis. It is evident that the GDH of the pollen differs from that of the plant, so that one may speak of isoenzymes. The GDH of the pollen is synthesized de-novo during the growth of the tubes. Furthermore, the GDH of the style is activated under the influence of the pollen tube growth which induces conformational changes in the GDH molecule. By joining the pollenand style extract, the GDH deamin. of the style is activated by an undialyzable activator from the pollen. The GDH from both pollen and style are NAD+ specific and have no alanine dehydrogenase activity.