Around 400 school students in Mykolayiv were evacuated after an unknown aerosol was sprayed there on the morning of 21 May, the regional directorate of the State Emergencies Service has said on its website.
There is no threat to the children's lives but 36 students were sent to hospitals with coughing, tearing nausea and throat irritation. Two were admitted into emergency care with dizziness and stomach ache, according to the deputy head of the Mykolayiv regional state administration, Yevhen Shevchenko.
He added there was no threat to their lives.
According to the Health Ministry, 39 children required medical help.
Police are treating the incident as an act of hooliganism.
On 8 May, children fell sick and some blacked out at an outdoor school event in Cherkasy. Several dozen children needed medical assistance.
The Health Ministry has so far failed to identify what had caused the poisoning.