I think MH lighting for herbs is a little overkill. It can be done cheaper and easier with fluorescent lighting-but if MH is what you desire, go for it! As for the photoperiod, I'd say like most plants, give it 16 hours or less. AFAIK, all plants need some "sleep".

Sam

hey hydro_rocket, basil will grow very well and very quickly under a MH light! if you leave it under 24 hours light it will flower automatically once it is mature..

to keep the plants growing bigger (vegetating) just pluck any buds off that you see before they become flowers.

if you take cuttings of the mature basil and root them they will start flowering almost immediately under 24 hrs light

When I tried this, they grew fine but tasted off. You might need to experiment with the temperature or fertilizer mix to get it right.

I have started thousands of basil plants from seed in a heated greenhouse using no other light than that provided by nature. If you are starting yours indoors and not in a g-house fluorescent lights should be used. After germination they certainly would profit from additional light in the Winter months when the sun is stingy but they will do fine without it. More light would speed up the maturation process no doubt.
If you want to grow mature basil in g-house during months of low light then you will have to use supplemental lighting and I would recommend sodium because they are more cost efficient. I have MH and they do make the old electric meter spin!
I purchased a helpful book this past Fall entitled..GROWING AND SELLING FRESH-CUT HERBS, Second Edition by Sandie Shores. She grows a lot basil and other herbs under cover and under lights on a commercial basis.

Bob

Thank you all for your posts! My main concern is the actual lighting change in hours going from 24/7 to natural hours; IÂm thinking that might "trigger" them into premature flowering. What do you think?

Thanks again,
Joe

Basil flowers most aggressively at 18/6, and gives the highest yield at 24/0.
DOI: 10.1007/BF02904661

Power requirements(watts) for a light fixture determine how fast your hydro meter spins, not the bulb type. A 400w HPS will spin your meter at the same speed as a 400w MH. The Latter has PAR at different wavelengths than the HPS, and being more similar to early summer sun, conventional wisdom is that it's better for vegetative growth, and HPS for flowering.
I've grown sweet basil with HPS and it works just fine.

Here is a link that might be useful: Peer reviewed article