May proved the most fruitful month this year for investors in Greek stocks. The Athens Exchange general index gained a cumulative 4.97 percent, closing at an 88-month high of 4,973.21 points (not seen since January 2000) on Friday. The positive mood in global bourses, the robust profitability of traditional blue chips (National Bank, Titan, Viohalco, OTE, Alpha Bank) in the first quarter, the revival of small-caps and capital inflows from abroad all contributed to the fine performance. It may be noted that a good number of shares have already attained historic highs, such as National Bank (44.38 euros), Piraeus Bank, Coca-Cola HBC, Folli-Follie, Hellenic Exchanges, Kloukinas-Lappas, and Fourlis. Staff of stockbrokerages are sounding the warning bell for a number of stocks of problematic firms, considering that their prices do not reflect fundamentals. Notably, the prices of several small-caps have risen steeply, either because they have long been undervalued, or because investors have now sharpened their appetite for risk in a bull market, chaneling part of their profits from blue chips and mid-caps into the smaller capitalizations. A number of analysts, however, take the view that barring the emergence of destabilizing factors, such as geopolitical upheavals or a further steep rise in the price of oil, the Greek stock market will continue reaping the benefits of the favorable sentiment abroad. The Athens Exchange on Friday brought into force the changes in operating regulations regarding the criteria and trading times for stocks in the under supervision category, the criteria for inclusion in the large capitalization category, price fluctuation limits and the method for calculating closing prices.