The 20-year-old actress/singer was filming scenes for Georgia Rule when she was overcome by the intense heatwave that has plagued southern California in recent weeks.
She was taken by a private car to a Los Angeles hospital for treatment.
According to her representative, Leslie Sloane-Zelnick, "She was overheated and dehydrated."
Sloane-Zelnick says the star spent two hours in the hospital after being given a Vitamin B shot.
She adds, "She was filming in 105-degree Fahrenheit weather for 12 hours."
A friend of the actress tells People magazine the night before her hospitalization, Lohan was partying at Hollywood nightclub Guy's.
An observer at Guy's confirms to the magazine, "She was definitely partying. She wasn't over the top, but having a great time."
The redhead started the year off in the hospital after a severe asthma attack on Jan. 2 left her gasping for breath and in need of medical attention. Later the same month she needed 10 stitches in her shin after falling on a broken cup at rocker Bryan Adams' home in London.
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