Judge John Kness said he would likely take about four weeks after the hearing ends to make a ruling. The case is being heard in federal court in Chicago.
The Federal Trade Commission filed a lawsuit on May 16 aimed at stopping the transaction in a rare move to block a large pharmaceutical deal.
The FTC said it opposed the deal because of concerns Amgen would leverage its big selling drugs to pressure insurance companies and pharmacy benefit managers to give favorable terms for Horizon's two key products - the fast-growing thyroid eye disease treatment Tepezza and gout drug Krystexxa.
(Reporting by David Thomas; Writing by Diane Bartz; Editing by Mark Porter)
