Promastigotes and amastigotes of Leishmania mexicana amazonensis, incubated in the presence of 20 ng/ml of 12-O-tetradecanoyl phorbol-13-acetate (TPA), an exogenous protein kinase C activator, developed several membrane and cytoplasmic alterations. Increased exocytic activity was observed especially in the amastigotes which had an enlarged flagellar pocket. Treatment with TPA induced protrusions of the plasma membrane where cytoplasmic elements (ribosomes and sub-pellicular microtubules) were not seen. Freeze-fracture replicas of TPA-treated parasites showed reduction in the density of the intramembranous particles (IMP), which were not seen on either fracture face of the membrane lining the protrusion. Cytochemical observations showed that sterols and anionic sites which bind to filipin and cationized ferritin particles, respectively, can be detected in the membrane lining the protrusions. However, the pattern of distribution of anionic sites, which bind colloidal iron hydroxide particles, and acid phosphatase in the membrane lining the protrusion region differed from the other portions of the plasma membrane.