"We all have to realize is that this debate is as old as the game itself," the Canadian prime minister said.
Harper has a book coming out next month on the early history of professional hockey. His unsolicited remarks Tuesday came from a beach-front hotel following a summit of Asia-Pacific leaders.
"Since I'm taking off my hat soon on this (hockey historian) business anyway, as you know, I welcome the opportunity to comment on it," he said at a news conference.
Harper said he's particularly concerned about shots to the head in hockey, adding he admires skill over brawn.
"I do think that authorities have historically not taken their responsibility to try and keep the rough, tough part of the game within the rules," he said.
"These are very serious issues and they do have to be taken seriously by the NHL and other sports bodies."
During last week's opening of the NHL season, Montreal Canadiens enforcer George Parros tangled with Toronto Maple Leafs tough guy Colton Orr. Parros fell hard on his chin, knocking himself unconscious, and is out indefinitely with a concussion.
The confrontation was one of several marring the early season, renewing again the debate over fighting on ice.
"I'm not trying to be nonchalant about some of these incidents, which I think are of concern to any parent watching this and seeing examples set and worrying about what could happen to their own boys and girls when they step on the ice," Harper said from a hotel penthouse overlooking the Indian Ocean.
The prime minister called the level of violence in pre-war hockey "quite shocking."
"There has never been an era in hockey, including from the very beginning, where violence was not an issue of controversy," he added.
Harper's book is titled "A Great Game: The Forgotten Leafs and the Rise of Professional Hockey." Author and Globe and Mail columnist Roy MacGregor was an editorial consultant. The book is to be released Nov. 5, with proceeds going to a military charity.