The mast cell activator, compound 48/80, produced increased vascularity and tortuosity of blood vessels in the chorioallantoic membrane (CAM) of ten day old embryonic chicks. Application of 400 micrograms/ml resulted in rapid mast cell granule release, observed after 1 min at both light and electron microscopy level, and resulted in the greatest increase (69%) in CAM mesenchymal vessels. The half-maximal dose was 38.6 micrograms/ml, computer-derived from the dose-response data. It is apparent that a single episode of mast cell degranulation is sufficient to induce vessel growth over several days, but the fact that a sublethal dose is required for maximum stimulation casts doubt on its biological significance. This pattern of response resembles that previously found by us with histamine and has not been found so far with commercial heparins and chemically modified derivatives. In contrast, a sublethal dose of porcine intestinal sodium heparin results in an antiangiogenic effect on the ectodermal capillary plexus.