The development of the transmitting tissue (TT) in pistils of diploid and tetraploid grapes was compared. Pre-bloom pistils of two diploid grapes, 'Campbell Early' and 'Muscat of Alexandria', and pistils of two tetraploid grapes, 'Kyoho' and 'Suiho', were cross-sectioned at intervals during 21 days before anthesis (IDBA). In mid-style sections the TT size and the number of TT cells per style were similar between the diploid and tetraploid cultivars, although the initiation of the TT began earlier in diploid grapes than in the tetraploid ones. In the ovaries of the diploid grapes, 3 to 5 layers of the cells in the outer septa at the position of the upper ovules became round and loosely packed, considered to be converted into TT cells 13 DBA. Similar cells, consisting of only I or 2 layers of the septum in the tetraploid grapes, were converted to TT cells in 9 or 6 DBA, depending on the cultivar. Ovary growth and pollen development during the 21 weeks before anthesis, however, proceeded faster in the tetraploid grapes than in the diploid ones. We concluded that the insufficient TT development in tetraploid grape ovaries is caused by an inactive conversion of septum cells into TT between 13 and 6 DBA, when the florets are growing actively.