Rao was rushed to the government hospital in the district headquarter town of Ongole. Doctors said his condition was stable, but did not immediately disclose the kind of poison used in the attack.
Rao had highlighted many irregularities in the use of government funds and earned the wrath of local officials and contractors in the district.
Last week, he had complained to the district administration about a threat to his life from a local official whose corruption he was trying to expose.
He had also claimed that despite his complaint, no action was taken against the official.