Tropical trees may change in pre-pollination pollen viability according to the types of pollinator, flower exposure, canopy position, plants' age and pollen predacious ants and thrips. A canopy study system was used and in vitro pollen viability measures, to evaluate relative impact of such factors in one forest tree species, monsoon forest teak, Tectona grandis Linn. of the family Verbenaceae. Relative pre-pollination pollen viability was measured with Alexander's stain, TTC, MTT stain and in vitro pollen germination. The impact of ants and thrips was gauged from bagged and open flowers, and direct counts on flowers in Tectona grandis; canopy position impact (affecting light, exposure, temperature) was gauged using collections made with pole clippers, and by reciprocal transfer of inflorescences on branches. A few native insect species were scored for viable pollen as they first left their nests in early morning. Viability decreased according to ants and thrips presence in Tectona grandis. Pre-pollination pollen viability was higher on the higher flowers. Pollen viability in transplanted inflorescences changed. Relative pollen viability was in agreement using different in vitro pollen viability tests. The frequency of pollen viability increased steadily from low to high canopy height. The percentage of viable pollen was different for different assays showing similar trend to increase. The middle and high canopy height did not show much variation in pollen viability compared to low canopies. So the tree canopy height may attain a vital role in determining pollen viability. The proportion of viable pollen, varied greatly, showing a steady increase with plant age, and the number of pollen grains per flower also increased with age. These results support the qualitative prediction that pre-pollination pollen viability gradually decreases with ageing, reflecting an increase in genetic load with age.