Mast cells secrete -

Mast cells are large cells found in areolar connective tissue, especially near the blood vessels.
Their cytoplasm is packed with granules that contain histamine, heparin and serotonin.
Histamine is a vasodilator: it dilates the blood vessels and increases their permeability, and it is released during inflammation and allergic reactions.
Heparin is an anticoagulant and serotonin is a vasoconstrictor.
Myoglobin is the oxygen-storing pigment of muscles, and haemoglobin is the respiratory pigment of red blood cells; hippuric acid is an excretory product.
So mast cells secrete histamine.