Hypericum perforatum or St. John's wort is a plant grown commercially for use as a medicinal plant. The consistency of secondary metabolites that act as active ingredients in H. perforatum preparations is a constant problem and is attributed partially to environmental conditions experienced by the plants during growth. Controlling the light provided to plants has been an effective way to manipulate plant growth in other crops. The optimal lighting conditions and time to harvest H. perforatum grown in controlled environments was the primary goal. The effects of light intensity, quantity and quality on biomass and secondary metabolites hyperforin, pseudohypericin and hypericin over time were investigated in four experiments. An additional experiment demonstrated that H. perforatum will flower under the long-day conditions (16 hours of light) used in all experiments. Light intensities from 90 to 340 ?mol m-2 s-1 were investigated while daily light integral was held constant. Effects of daily light integral were demonstrated by holding the light intensity constant and varying the light integral from 8.6 to 20 mol m-2 d-1. The response of metabolite production to the presence or absence of UV-A and UV-B was also explored. Finally, the usefulness of stressing the plants with supplemental UV-B light just prior to harvest was determined. Results showed a very small or no significant increase in the secondary metabolites quantified in response to increasing light intensity, light integral or the addition of UV-A or UV-B light. Biomass production was shown to increase with exposure to increased light intensity and light integral. It was demonstrated that all of the metabolites increased their concentrations as plants transitioned from a vegetative to reproductive state. For growth in controlled environments, increased light integral did increase metabolite production indirectly as biomass increases led to a more rapid time to flowering. Since metabolite concentrations were shown to rise so dramatically when plants were flowering, the best protocol for maximizing metabolite production per square meter of growing space is to furnish plants with as much total light as possible which would hasten the time to flowering, then harvest plant material at the full bloom stage.