Dempster (5-5) went six innings for his fourth win in his last five decisions -- and first win in five decisions as a visitor at the ballpark he used to call home.
"I didn't feel it when I got up," Dempster said. "I have never had anything like this. It was tightness in my hip. I couldn't get loose. It wasn't a lot of fun warming up. It was hard to push off. I took a little break. It was hot out there and it loosened up as the game went on."
The right-hander, who pitched for the Reds in 2002 and 2003, allowed three hits and one run with one walk and eight strikeouts to break a tie with Kirk McCaskill for the second-most major league wins by a Canadian-born pitcher. Dempster has 107, behind only Hall of Famer and former Cubs ace Ferguson Jenkins (284).
Rodrigo Lopez nearly got the call to start instead.
"I can't say enough about the pitching," Cubs skipper Mike Quade said. "Ryan had some hip tightness. I was wondering what to do with the lineup card. I told Dusty and asked the umpires. They told me I had until he took the mound. He just couldn't get his hip loose. He pitched so well. He got better as the game went on. It was nothing serious."
Marmol blew his last save opportunity, prompting angry comments from teammate Carlos Zambrano. Quade wanted to get him back on the mound as soon as possible but the Reds won the previous two games handily.
"It was good to get Marmol out there to get his confidence back," Quade said.
Reds starter Bronson Arroyo (4-6) lasted 6 1/3 innings on the sunny, steamy afternoon, giving up eight hits and four runs -- three earned -- with one walk and three strikeouts as the Reds finished a nine-game homestand.
The game-time temperature was 90 degrees.
The Reds led 1-0 when Blake DeWitt, bumped up to the third in the batting order for the first time this season, reached on shortstop Paul Janish's error with one out in the fourth. Pena followed with a 443-foot home run, his ninth of the season.
"It's the sweetest feeling. It's right on the sweet spot on the bat, and not only that, it's the timing. It's perfect timing. It never gets old," Pena said.
Ramirez hit the next pitch 342 feet into the left field seats for his third homer of the season and 27th of his career against Cincinnati.
The Cubs have been plagued by missed scoring opportunities during the losing streak and stranded 10 more runners today. The home runs were more than welcome.
"Instant offense, runs like that are great," Quade said. "We still left some people on but overall we played great."
The Cubs added a run in the seventh when, with Kosuke Fukudome on third and Pena on first with two outs, reliever Jose Arredondo whirled and faked a throw toward an uncovered first base. He was called for a balk, allowing Fukudome to score and Pena to go to second.
The Reds loaded the bases with one out in the first inning, but Dempster coaxed Miguel Cairo -- who hit a grand slam Tuesday night -- into a popup before Fred Lewis grounded out to the end the threat.
They took a 1-0 lead in the third. Drew Stubbs led off with a soft liner to center for a single, stole second and moved to third on Brandon Phillips' groundout to second. Joey Votto struck out, but Jay Bruce lined a 3-2 pitch to right field for an RBI single.
The Cubs head home in a better mood with the win. The internal strife of the weekend is healing.
"We are going to enjoy this," Pena said. "We will have a nice trip home. We have selective ears in here (the clubhouse). We have the parental controls on the TV."
Two Cubs on the disabled list, outfielder Reed Johnson and infielder Jeff Baker, are scheduled to start rehab assignments Friday with Triple-A Iowa, and outfielder Alfonso Soriano is due to join them Sunday, Quade said. ... Reds right-hander Jared Burton, who hasn't pitched this season while dealing with shoulder inflammation that required surgery, will report to Cincinnati's Goodyear, Ariz., complex on June 12, manager Dusty Baker said. "It's like spring training all over again for him," Baker said. ... Reds third baseman Scott Rolen missed his fourth consecutive start with a viral infection in his throat and not strep throat, Baker said, quoting a team doctor.