BUENOS AIRES, May 21 (Reuters) - Argentina's stock, currency and bond market trends on Thursday:
* The MerVal stocks index .MERV rose for a fourth straight session due to selective buying, bucking the global trend and reversing midday losses to close up 0.69 percent at 1,560.85 points -- its highest level since October.
* The index is up 22.4 percent since May 1 and has risen 8.5 percent this week.
* Energy group Pampa Energia PAM.BA rose 9.4 percent to 1.16 pesos per share and MerVal heavyweight Tenaris (TENA.BA), which makes steel tubes for the global energy industry, gained 0.73 percent to 55.6 pesos per share.
* On the broader stock market, volume was thin at $16 million. Among active issues, 26 advanced, 30 declined and 20 ended unchanged.
* Prices for government bonds traded over-the-counter in Buenos Aires fell 2.0 percent on average on profit-taking after five sessions of gains that saw an average rise of 8.7 percent.
* The losses were led by peso-denominated Par paper ARPARP=RASL, which fell 4 percent, according to the ask price.
* In formal exchange between banks, the Argentine peso closed at its weakest level in seven years, falling 0.13 percent to 3.735/3.7375 per dollar ARS=RASL due to central bank dollar purchases aimed at boosting exports, traders said.
* In informal trade between foreign exchange houses, as measured by Reuters, the peso dipped 0.07 percent to end at 3.795/3.8 ARSB=.