The quantitative effects of temperature and light intensity on accumulation of phenolics were examined on greenhouse-grown plants of Hypericum perforatum L. Plants were grown in a greenhouse separated into two parts: shaded by 50% transparent polyethylene cover and un-shaded. Temperature values and light intensities were measured daily during the experiment, while plants were harvested weekly for HPLC analyses. Multi regression analyses were performed to describe the quantitative effects of temperature and light intensity on phenolics accumulation. According to the results, increases in temperatures from 24 degrees C to 32 degrees C and light intensities from 803.4 microMm(-2)s(-1) to 1618.6 microMm(-2)s(-1) resulted in a continuous increase in amentoflavone, apigenin-7-glucoside, cholorogenic acid, hyperoside, kaempferol, rutin, quercetin and quercitrin contents. The relationships between temperature, light intensity and phenolics accumulation were formulized as P= [a + (b1 x t) + (b2 x l) + [b3 x(t x l)]] equition, where P is the content of the corresponding phenolic, t temperature (degrees C), l light intensity (microMm(-2)s(-1)) and a, b1, b2 and b3 the coefficients of the produced equation. The regression coefficient (R2) value for amentoflavone was 0.84, for apigenin-7-glucoside 0.87, for cholorogenic acid 0.83, for hyperoside 0.95, for kaempferol 0.76, for rutin 0.70, for quercetin - 0.93, and for quercitrin - 0.86. All R2 values and standard errors of the equations were found to be significant at the p<0.001 level. The mathematical models produced in the present study could be applied by Hypericum researchers as useful tools for the prediction of phenolics content instead of routine chemical analyses.