almost 10 years since the project was first discussed.
The first one stands just to the south of Beauly, and before the end of this year another 133 will be erected. This will take the line to Fort Augustus, with power flowing by the end of the year.
By October 2014 the 137 miles to Denny should be completed.
Those behind the project say it should create 1500 jobs.
After the foundations were completed, the tower took five days to piece together on the ground, and another two days to bolt together all the different parts, using a large crane to lift the parts into place.
"It really is just large-scale Meccano," one engineer explained, as four men climbed the pylon legs in about 10 minutes yesterday to fix the final 32.8ft (10 metres) of the galvanised steel structure in place.
It stood at 161ft, towering over the 50-year-old neighbouring 92ft pylon.
However, many of the pylons further south will be even taller at almost 200ft (60m), which has outraged people concerned at the impact on the Scottish landscape.
The 600 new pylons will replace 850 of the older ones.
Ian Funnell, SSE's Director of Transmission, said: "Work on the replacement line is vital in order to upgrade the grid to accommodate further onshore wind, new offshore wind and emerging marine generation in the north of Scotland.
"We should be erecting two or three towers a week in the early part of the summer and up to three times that by late summer."
He said the volume of electricity generated from wind farms transmitted by the SSE subsidiary Scottish Hydro Electric Transmission's network had exceeded one gigawatt (1000 megawatts) for the first time on February 14.
However, the power line would be able to carry twice that, enough to power a sizeable city.
He said: "We are working hard to engage with local communities and interested stakeholders and looking at opportunities for longer-term benefits such as local employment.
"We will have nearly 500 people working this summer because we will have wiring squads each 70 strong.
"We will recruit youngsters and put them through training programmes.
"We are starting in the next couple of months. But, after 10 years, today is a very important milestone."
However, Helen McDade, head of policy for the John Muir Trust, said: "This is a day for reflecting on another piece of Scotland's unique and beautiful landscape lost to development.
"The impact of the overhead line through the central Highlands will soon become apparent just as the useful contribution of large-scale wind power in north Scotland is being increasingly questioned.
"On top of that, the Beauly to Denny line is now going to cost £600 million, twice the figure given at the public inquiry.
"It's clear, as we suggested, that a subsea cable should have been properly considered.
"This is just the start, and further major transmission capacity expansion is currently being passed without adequate environmental impact assessment. It's time for our decision-makers to learn lessons and protect our valuable natural heritage."
Work began last year to remove the existing pylons in the Cairngorms National Park. Some 33 miles of old line or wirescape is to go between Boat of Garten and Cairnmore.