The second larval instarof the cotton leafworm, Spodoptera  littoralis (Boisd.) as a series polyphagous pest was treated, in laboratory, by ingestion of two chlorophyll derivatives namly copper chlorophyllin (Chl-Cu) and magnesium chlorophyllin (Chl-Mg) to study the possible role of mixed function oxidases (MFO) in detoxifying the dye, and determine the efficancy of treatment on protein metabolizing enzymes or transaminases. Moreover, to clarify whether the photoactive dyes have non phototoxic effect. The larvae were treated for 24 hrs with LC₅₀ of the dye followed by exposure to direct sunlight for 2 hr. Copper chlorophyllin was more potent than magnesium chlorophyllin and represented by LC₅₀ = 0.130M and LC₅₀ =0.299 M, respectively. The percentage mortility of larvae treated by LC₅₀ of the dyes and kept in  dark during the experimental time was up to 9%. The dark process appear to be less active than the light induced photo dynamic reactions. Biochemical analyses revealed that treatment led to induction of MFO system. It was 733, 926 and 687 μ mole substrate oxidized /min/g. b. wt for Chl-Mg, Chl-Cu and control, respectively. Effect of treatment on transaminases showed that  GPT activity was significantly changed by both chlorophyll derivatives, while GOT activity was non significantly changed. Induced GPT level could be considered  as a sing or secondary effect of chlorophyllin poisoning. This effect has been interprreted as suggestive of interinsic capacity of insect to repair damage. MFO might share in detoxifying chlorophyllins, and development of resistance to photodynamic pesticides. The observed dark effect suggests that the used photosynthetizers posses low interinsic chemical toxicity.