A spokesman for Tymoshenko said the loss of Rybakov and But, who support market reforms and closer Ukrainian relations with the European Union, would not affect the functioning of the government.
Ukraine has experienced continued political volatility since the Orange revolution uprising led by Yushchenko, the president, and Tymoshenko, who have both vowed a pro-Western course including Nato membership.
Despite their Western tendencies, since then both politicians have repeatedly fallen out, while pro-Russian politicians have fought to resist their Westward policies.
Recent weeks have seen repeated rumours that Tymoshenko might leave her post as she prepares for an expected presidential run against Yushchenko in elections due either next year or in 2010.
Ukraine's ruling coalition was formed in a knife-edge vote last November after early parliamentary polls in September.