An investigation was carried out with paclobutrazol (PBZ) in order to study its effect on the photosynthetic and anatomical characteristics of Catharanthus roseus (L.) G. Don. The chlorophyll content and related parameters like net photosynthetic rate, transpiration rate and internal CO2 concentration were measured from control and treated plant samples. The anatomical characteristics, viz., leaf thickness, epidermis and cuticle, palisade layer, spongy layer and vascular bundle were studied. The root alkaloid ajmalicine was extracted and estimated from both treated and control plant roots. Photosynthetic pigment increased under PBZ treatment. PBZ treatment increased the net photosynthetic rate and internal CO2 concentration, but reduced transpiration. PBZ treatment increased the thickness of the leaf, epidermis and cuticle, palisade layer and spongy layer but reduced the diameter of xylem vessels significantly when compared to control, however the phloem elements had shown an increased diameter as compared to control in the PBZ treated plants. There was a significant enhancement in ajmalicine content under PBZ treatment when compared to control plants. The results suggested that the application of PBZ have significant effects on photosynthetic and anatomical responses thus can be used for improving productivity in medicinal plants. (C) 2007 Elsevier Ltd. All rights reserved.