The transpiration experiment was done under greenhouse conditions with a C3 plant sweet pepper(Capsicum annuum Linn.)and two C4 plants,sorghum(Sorghum bicolor L.Moench)and maize(Zea mays Linn.).Three species were irrigated with three different water treatment levels of 100%,66%and 33% which gave a comparison of tolerance and adaptation to irrigation and two different levels of water stress.The measurements of transpiration rate and stomatal conductance were done between 8.00 a.m.and 16.00 p.m.with measurements about each 1.5 h with an infrared gas analyzer.The results showed that Z. mays probably due to a hiigher leaf area had very low values and was significantly different(LSD pairwise comparison)from C. annuum and S.bicolor.The hypotheses that C4 plants and C3 plants have different transpiration rates and stomatal conductance could not be shown with the results.However,the hypotheses that for the same species,the highest values in transpiration rate and stomatal conductance were with the 100% irrigation treatment and the lowest values were with the 33% irrigation treatment could be accepted due to the results of this trial.