Before light rain began to fall, 29 cars had turned 697 laps. The practice session ended at 5:03 p.m. -- 1 hour, 57 minutes short of its scheduled length.
The 29 cars that took the track were the most for an Indy 500 Opening Day since 1999, when 39 cars turned laps. The last time Opening Day was affected by rain came in 1997, when the practice session was completely rained out.
Scott Sharp had the fastest time, clocking 229.764 miles per hour in a Dallara/Toyota.
"It was a pretty compacted afternoon with the rain," Sharp said. "I think everybody got some help, some drafting help. Obviously, I did. I think we have a really good car. We have a strong baseline. I don't know how much we can take from a day like today, but this is certainly a start."
Sharp was the most recent winner on the IRL circuit when he triumphed at Twin Ring Motegi in Japan on April 13.
"I think for us it's a little bit different this year with so many changes to our team in the offseason, personnel-wise, and our switch to Toyota, we felt like it might have taken a little time for it to all come together," Sharp said. "That win pumped us all up and gives us a little more confidence to get a victory at Motegi. If we do our work and our job here, there is no reason why we can't win this race."
Two-time defending Indianapolis 500 winner Helio Castroneves was second at 228.088 mph in a Dallara/Toyota.
"We did a test a couple of weeks ago and that seemed to help us get faster and comfortable," Castroneves said. "We were able to get up to speed right away and the conditions were pretty much the same as a couple of weeks ago. I think our time will be much faster when we get closer to next weekend. But right now, it's very fast for Opening Day."
Tora Takagi of Japan was third at 227.884 mph in a G Force/Toyota and turned the most laps in the session, taking 57. This is Takagi's first Indianapolis 500.
"I cannot complain about being third-fastest at Indy, no matter what day it is," Takagi said. "I love the fast corners very much. I feel very comfortable in the car. There were no dramas today."
Another rookie at Indy is Dan Wheldon of Britain, who had the fourth-best time at 227.716 mph in a Dallara/Honda. Michael Andretti, who will retire after the May 25 race, rounded out the top five at 227.461 mph.
After Opening Day ceremonies concluded, Felipe Giaffone was first on the track, followed by two-time IRL champion Sam Hornish Jr., Billy Boat and Jaques Lazier. Giaffone turned the first official practice lap for Toyota at the Indianapolis Motor Speedway.
Sarah Fisher's blown engine brought out the first yellow flag just 19 minutes into the session.
"An oil-fitting came loose and dumped a bunch of oil all over the inside of the car, but everyone including my dad and I pitched in and cleaned it up so we could get back out on the track," Fisher said. "As we went out to run again, the track went yellow for rain. Sometimes, you just have days like that; it's something we can't control, but we're hoping to get our share of these little issues out of the way early in the month."
Ohio crash that killed two caught on camera [VIDEO]
Mena Suvari shares her delightfully awkward Christmas card photo