Sarah Jessica Parker Wigs Out on 'Lovelace' Set ... After Demi Moore Drops Out

Sarah Jessica Parker was hard at work on the L.A. set of "Lovelace" on Wednesday ... taking over the role Demi Moore vacated after she was hospitalized last week.

Parker is rocking a long blonde wig to play '70s feminist icon Gloria Steinem in the biopic of late porn star Linda Lovelace.

As we previously reported, Demi dropped out of the project in the wake of her medical emergency ... when she inhaled dangerous amounts of nitrous oxide in the form of whip-its, lapsed into semi-consciousness and experienced seizure-like symptoms.