NEW YORK ( TheStreet) -- Stocks closed lower on Thursday, after posting gains earlier in the session, as investors continue to worry over Greece's ongoing debt woes.
The S&P 500 lost 0.30%, the Dow Jones Industrial Average fell 0.42%, or 75 points, and the Nasdaq gave up 0.20%.
After the closing bell, Valeant Pharmaceuticals (VRX) offered to acquire Zoetis (ZTS), an animal-health company, sources told The Wall Street Journal. Valeant rose 0.5% in after hours trading, compared to Zoetis' 1.6% gain. Plus, athletic retailer Nike (NKE) reported earnings per share of 98 cents after the closing bell, beating Thomson Reuters estimates of 83 cents. The stock rose 2.5% following the markets close.
"Concerns around Greece seem to be rising," said Ian Winer, head of equity trading at Wedbush Securities. "But in general it's not a huge move and I wouldn't read too much into it."
Aside from Greece, the Dow Jones Transportation Average fell 0.84% on Thursday, reaching correction territory, falling over 10% since its November 2014 high.
It was a busy day for health stocks. Shares of Humana (HUM) jumped after reports surfaced that Aetna (AET) was close to acquiring the health care giant.
Bloomberg reported Aetna was closing in on an acquisition of Humana and could reach a deal as early as this weekend. Humana shares closed ahead 7% after being halted earlier for volatility.