ABSTRACT Imazapyr (IM) treatment applied at early reproductive developmental stages have been associated with induced male sterility in sunflower (Helianthus annuus L.). The aim of this work was to determine the effects induced by two different doses of IM-treatment (1X and 2X; 80 and 160 g a.i.·ha-1) on pollen grain yield, pollen viability and on the exine surface, in an intermediate resistant (I) and a completely resistant (R) genotype. An anatomical analysis of some stages during the microsporogenesis and microgametogenesis was also achieved by confocal microscopy. Plants of both genotypes survived the herbicide treatments and showed no external injury symptoms. Differential response to different doses of IM was obtained in R and I genotypes. The pollen grains per flower and the viability in seeds were significantly reduced in the I genotype by the 2X treatment; however, the pollen viability of the remaining grains was not altered, and the diameter of the pollen grains was increased by the herbicide treatment. Anatomical and cytological observations of the sporogenous tissue development were made via confocal microscopy. IM treatment induced alterations in sporogenous cells during the initial phases of microsporogenesis in both genotypes, and the 2X dose accelerated the microgametogenesis process, particularly in the I genotype. These findings improved the understanding of the mechanism underlying the gametocide effects of the imidazolinones in sunflower.