Kerry Kennedy has plead not guilty to driving under the influence of drugs during her July 13 car crash ... claiming she had a seizure as a result of an old head injury.

Kennedy appeared in North Castle Town Court in New York last night to answer for the incident in which she crashed into a tractor-trailer around 8 AM. Cops said she failed multiple sobriety tests and exhibited slurred speech.

But Kerry says she may have accidentally told the cops she had taken an Ambien sleeping pill that day ... but says she was mistaken, claiming tests showed "no alcohol, recreational drugs or prescription medication in my system."

Kerry also claims she underwent neurological tests following the crash ... and an MRI revealed "an area of hyper density" in her brain which she claims is the result of an old head injury. 
 
"This accident was caused not by a sleeping aid but by a complex partial seizure," Kerry said.

Kennedy -- daughter of Robert F. Kennedy -- is due back in court in August.

Kerry Kennedy's car crash and subsequent DWI arrest might have had nothing to do with drugs and instead could have have occurred because she suffered from a seizure -- this according to a new report.

Kennedy was arrested Friday AM after driving erratically and crashing into a tractor trailer on I-684 in NY. Police charged with driving while impaired by drugs after she allegedly admitted to taking an Ambien.

But a source tells the NY Post that the Kennedy family now believe the 52-year-old may have suffered a seizure instead. The source says, "She does not remember how she got to the side of the road. All she remembers is feeling very woozy."

The source adds that the Kennedy family now fears an undiagnosed medical condition may be to blame.

Following the incident, the source claims Kennedy's blood and urine tests (taken at Northern Westchester Hospital) showed no signs of drugs or alcohol. But the cops are still waiting on their own blood tests, which should be available shortly.

Robert F. Kennedy's daughter Kerry Kennedy was arrested in New York this morning after cops say she was driving while under the influence of drugs ... and crashed into a tractor-trailer.

The State Police released a statement saying they received 911 calls around 8:00 AM reporting a woman in a 2008 Lexus who was driving in an "erratic manner."

When cops arrived to the scene ... officers found Kerry's damaged car nearby ... with Kennedy seated behind the wheel.

Officers approached the vehicle and determined Kennedy was under the influence of drugs.

According to WABC in NY, Kennedy told the officers she had taken an ambien -- a powerful prescription sleeping pill.

52-year-old Kennedy was arrested and charged with driving while impaired by drugs.