This study was conducted to evaluate the effects of root applications of gibberellic acid (GA3) on photosynthesis in tomato plants grown hydroponicaUy. Photosynthetic rates (mg COs/dm 2/hr) determined using an open infrared CO2 gas exchange system showed a 40-50% increase within 5 hr after treatment with a 1.4 ~M gibberellic acid (GA3) to their roots. The effect was shown to persist for the duration of the experiment (9 days). Plants receiving pulses of 1.4~tM GA3 to the roots for 1, 4, 8 or 12hr exhibited significantly higher photosynthetic rates than the control for 6 days following treatment. By day 9 however, there was no significant difference. Continual treatments with 1.4 vM GA3 to the roots maintained the photosynthetic rate significantly higher than the control for the duration of the experiment. Interestingly, at the lower light levels the percent stimulation was more dramatic. There was approximately a 90% increase in the photosynthetic rate at 80 ~E m-2 s-1 while at saturating light conditions. (560 ~tE m -2 s -1 ) there was approximately a 40% increase over the control rate. The light saturation point for both treated and control plants was 240t~Em -2 s -1 . Applications of physiologically relevant concentrations of GA 3 to the roots of tomato plants stimulates photosynthesis more consistently than that achieved by previous studies involving foliar absorption.