On April 23, 1999, Fernando Tatis became the first player in the history of Major League Baseball to hit two grand slams in one inning as he led the way in an 11 run outburst during the third inning of a 12-5 win over the Dodgers in Los Angeles. The Birds found themselves trailing 2-0 going into that third frame before Tatis earned himself a spot in the record books. Both grand slams came off of pitches delivered by Chan Ho Park. The first came on a 2-0 pitch with no outs, and the second came with two outs on a 3-2 pitch that instantly made history as it sailed over the left field wall.
Watch the historic moment here: http://m.mlb.com/video/v3297525/042399-tatis-hits-two-grand-slams-in-one-inning
Check out the box score here: http://www.baseball-reference.com/boxes/LAN/LAN199904230.shtml