Shoot tips of the diploid rose Thérèse Bugnet were treated in vitro to oryzalin at concentrations of 5 and 15 microM. Tetraploid shoots were obtained in highest frequencies (40%) after exposure to 5 microM oryzalin for 14 days. Thin (1 mm) nodal sections were treated with 5 microM oryzalin and the highest frequency of tetraploids (66%) was obtained after exposure for only 1 day. The shorter exposure times required to induce chromosome doubling in thin nodal sections is attributed to the more efficient delivery of oryzalin to the meristem. Tetraploids were obtained from four diploid roses and hexaploids from two triploid roses. Chromosome doubling was accompanied by increases in thickness and a darker green colouration of the leaves and, in all diploid to tetraploid and one triploid to hexaploid conversion, the breadth/length ratio of leaflets was significantly increased. Internodes were longer in tetraploids than diploids but significantly shorter in hexaploids than triploids. The number of petals per flower in the tetraploid form of Thérèse Bugnet was double that of the diploid. Significant increases in pollen viability accompanied chromosome doubling of all four diploids and one of the two triploids.