From 00.00 April 7th, 2014 the State Border Service of Ukraine initiated verification of the citizens of the Russian Federation  requirements for length of staying in Ukraine.

Pursuant to the request from the Cabinet of Ministers of Ukraine on April 7, 2014 Russian citizens should follow the requirements of subparagraph 2 p.2 Order of extension the period of temporary stay of foreigners and stateless persons in the territory of Ukraine, approved by the Cabinet of Ministers of Ukraine of 15.02.2012 № 150.

This decision was done in response to the restrictions imposed by the Russian side, which, since January 1, 2014 , established a similar length of stay in the territory of the Russian Federation for the citizens of Ukraine.

So, from 7^th of April citizens of the Russian Federation, who have lawfully arrived in Ukraine, may temporarily stay in its territory not more than 90 days within 180 days from the date of first entry, unless another term is not defined by international treaties of Ukraine.

The effect of this decision will not apply to the citizens of Russia, crossing the Russian - Ukrainian border in the framework of the Agreement between the Cabinet of Ministers of Ukraine and the Russian Federation by residents of border regions of Ukraine and the Russian Federation of 11.10.2011 year.

http://dpsu.gov.ua/ua/about/news/news_3777.htm