Petunia has a monofactorial gametophytic self-incompatibility system which is characterized by a mechanism for pollen-tube rejection after incompatible pollination about half-way between stigma and ovule. From in vitro experiments it is known that pollen tube extension requires de novo synthesis of proteins on 80 S ribosomes. The synthesis of these structural proteins has no need of new mRNA formation. The situation in the style is quite different: both RNA and protein synthesis are positively affected by pollination. There is a qualitative difference in synthesis between self- and cross-pollinated styles. These findings suggest that inhibition or rejection of pollen tubes is 'the normal reaction', whereas unaffected penetration is based on the synthesis of special enzymes after compatible pollination which break down the existing incompatibility barrier.