It was long assumed that visible (≈ 380 ∼ 650 nm) and near infrared (I.R.) (≈ 650 nm ∼ 1000 nm) light does not interact with tissue. In the past three decades, however, it has been found that lasers in the visible and near I.R. cause biostimulation of injured tissues at very low levels of incident energy. We therefore studied the effect of various light sources in the visible (540 nm, 630 nm,) and near I.R. (660 nm, 780 nm, 940 nm,) on fibroblasts and keratinocytes. We found that at certain low energy doses there is an enhancement in their proliferation. Since these two radiation regions differ dramatically in their photochemical and photophysical properties, we share the opinion that visible light starts the cascade of metabolic events after being absorbed by endogenous porphyrins in the mitochondria, whereas infrared radiation starts the cascade of metabolic events by activating enzymes in the membranes. In order to examine our hypothesis that biostimulation in the visible is a photosensitization process, we measured the proliferation rate of fibroblasts enriched with very small amounts of hematoporphyrin derivatives (HPD) after HeNe (633 nm) irradiation and 940 nm irradiation. We have found that the effect of HeNe light on fibroblast proliferation was dependent on exogenous HPD while that of 940 nm was not. These results have important implications in both phototherapy and photodynamic therapy (PDT).