Lee County Sheriff Jim Johnson said Phyllis A. Raper, 57, of Guntown, was found dead in her cell around 8:45 p.m.
The inmate was prescribed several medications and when the jail staff brought it to her at 8:30 p.m. Sunday, she refused to take it.
Johnson said jail staff was alerted she wasn't responding or breathing and they conducted CPR but could not revive the woman.
The body was sent to Jackson for an autopsy. Preliminary autopsy results show there was no trauma but toxicology results and a more thorough autopsy report is expected to reveal more about the cause of death over the next few weeks.
The death is being investigated by the Mississippi Bureau of Investigation.