The synergids of Petunia show a number of changes in size, shape and ultrastructure during anthesis. Before anthesis, the immature synergids are small cells with a rather simple ultrastructure. During anthesis, they greatly enlarge. Between the tops of the synergids a filiform apparatus is formed. The numbers of mitochondria and dictyosomes increase, whereas the number of plastids does not. The mature synergids contain an extensive rough ER. From the ultrastructural data, the author concludes that the metabolic activity in the synergids increases sharply during anthesis. The mature synergids are proposed to produce and secrete substances which direct the growth of the pollen tube.