"I feel a lot more free and easy," he said. "I feel like the life on the ball, it's definitely there. I think it was there more than it was before. I learned a lot about my body and my shoulder going through the process consecutive times, and I think the biggest difference this time around is I trust it a lot more. It feels a lot stronger, I have a lot more confidence in it, and I think that's allowed me to get back to my natural arm slot and get on top of the ball and throw harder."
A's manager Bob Melvin took notice.
"That's the best bullpen I've seen him throw," Melvin said. "I think he feels psychologically a little bit better about letting it go. It seemed to me that he was guarding it a little bit more so the last time. He definitely was not this time. It seemed like there was a lot more whip."
Doolittle threw primarily fastballs, repeating his delivery and maintaining his mechanics with ease, and he also mixed in his changeup and slider, secondary pitches he has focused on during his first rehab experience.
He'll throw another bullpen Saturday and, barring any setbacks, could be near a Minor League rehab assignment soon after.
The left-handed closer, who saved 22 games for the A's in an All-Star 2014 campaign, is anxious to do his part, if only for a brief time, in 2015.
"After today, I'm more eager to move it along because of how well it went," said Doolittle, who later added, "My goal the whole time has been to come back and pitch this year. It's starting to look like that's something that is definitely a very realistic goal if everything continues to progress the way it has."
"It is important for him to get out there and pitch some innings and feel good about what he's doing," Melvin said. "It's one thing to just throw a bullpen and your arm feels good. It's another thing to get out there in games and look and see what the results might be."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.