First, let's start with some definitions. Agonists are chemical agents which stimulate a certain kind of response. Usually agonists are compounds which activate a receptor in cell membranes.
Antagonists are chemical agents which work against a certain kind of response.
Antagonist compounds may block a receptor so that it cannot be activated, or they may activate a different receptor in the same cell and activate a process which works against the process connected to the first kind of receptor. Receptors are protein-based structures embedded in cell membranes which serve as relays for signals. External chemical agents such as hormones or prostaglandins encounter the receptors on the outside of the cell. Secondary messengers then start up particular enzyme reactions inside the cell. Specifically these are the chemicals epinepherine and norepinepherine.