After three seasons on NBC, Star Trek was cancelled in 1969. As is often the case, the series was placed in syndication soon thereafter and famously became quite popular. The following was included in the March 10th, 1969 edition of Broadcasting:
Star Trek, coming off NBC-TV at the conclusion of the current season, will be placed in domestic syndication by Paramount Television, producer and distributor of the series. The package will include 70 episodes of the science-fiction series, which will have had a three-year network run .
A total of 79 episodes of Star Trek were broadcast by NBC between 1966 and 1969. I'm not sure why the syndication announcement mentioned only 70 episodes. It's possible that some were initially held back from syndication but I find that unlikely. Paramount Television placed a full page advertisement in Broadcasting a few weeks later promoting the availability of Star Trek for national syndication. And over the next year, three additional full page advertisements were placed in Broadcasting touting the success of Star Trek.
I included one of these advertisements in my A Look at Star Trek article but here I have all four of the Broadcasting advertisements for your enjoyment. Click on any of the images for a larger version.
Sources:1 "Program Notes." Broadcasting. 10 Mar. 1969: 66D.