St. John's wort (Hypericum perforatum) is currently used medicinally to treat neurological disorders, while research continues to seek practical methods to harness proven potential of this plant as an anti-cancer and anti-retroviral drug source. More than other medicinal plant preparations, bioactive components of H. perforatum are often found to vary by a factor of two compared to concentrations reported on labels for the prepared drug. This is a serious problem for medical researchers, physicians and consumers. Variability is attributable to fluctuations in environmental conditions to which the plants were exposed during growth and development. Growing H. perforatum in controlled environments, such as the greenhouse or growth chamber, can remove wide variations of common variables such as temperature, insect and disease pressures, and water status. Furthermore, plants may be exposed deliberately to stressors known to elicit increases in secondary metabolite concentrations. High light intensity has been shown to increase metabolite production, for example. Daily light integral control has been shown to produce predictable, consistent biomass gain in other crops but has not been related yet to secondary metabolite production in H. perforatum. This project focused on production of three important secondary metabolites, hypericin, pseudohypericin and hyperforin, from plants grown in floating hydroponic systems in controlled environments at light intensities of 90, 160 and 340 μmol*m-2*s-1 (at 25°C, 16 h photoperiod) which was a constant daily light integral of about 5 μmol*m-2*s-1. Plant growth data and metabolite concentrations (determined by HPLC), from seedling stage to 104 day old plants, are presented. Hypericin and pseudohypericin concentrations did not show any trend over time. Hyperforin concentration showed a steady increase over time and the 90 μmol/m2/s treatment was consistently higher than the other two light intensities. An exponential model for biomass estimation per square meter of growing space is presented, valid between the light intensities of 90 and 340 μmol*m-2*s-1, a photosynthetic period of 16 h, 25°C constant air temperature and ambient CO2 concentration.