''SELL in May and go away. Come back on St Leger day (September 11).'' It's an old British stock market adage that holds a little water in a number of equity markets, but analysts strongly advise against banking on the adage in Hongkong. ''Hongkong's not a seasonal market,'' said Mr Danny Truell, S. G. Warburg Securities head of research in Hongkong. Other markets, however, would seem to loosely adhere to the rule, at least the sell-in-May part. When to buy varies again from market to market. According to the Morgan Stanley Capital International Index the average return worldwide April-September since 1982 is 1.54 per cent. Throughout the rest of the year the return is 10.72 per cent. ''Many markets tend to be lulling in summer,'' agreed Mr Chan Wah-man of Pierson Securities. Research produced by S. G. Warburg suggests a similar trend in a number of markets throughout the world. In absolute terms markets in France, Germany, Italy and the UK have fallen on average in May and June since 1970. And most of the world's markets tend to show a downward trend during their summer months. In the UK, for example, the strategy would have worked 11 times since 1970, according to S. G. Warburg. During this period, the UK underperformed its trend by 6.2 per cent on average. Reasons vary from holiday times to the timing of corporate actions. Part of the reason for the inactivity in the UK, for example, stems from the busy social season such as horse races at Ascot, boat races at Henley, and tennis at Wimbledon. Warburg also points to more sophisticated reasons for the trend, which include the pattern of institutional cash flow tending to occur at the beginning of each calendar year, and also the timing of rights issues. But the volatile market in Hongkong never seems to take such seasonal rests, even if it too has tended to slip a bit in August. ''There's no reason why it should,'' said Mr Truell. In fact, the stock market tends not to underperform for more than two consecutive months throughout the entire year. This is not to say, however, that no patterns emerge. In fact, like many other markets worldwide, from December to the end of February the market performance is quite strong. According to figures produced by Millisis Financial Systems over the last 14 years the Hang Seng index has finished up in 11 of the years and down in only three. Similarly, in December over the last 13 years, the Hang Seng has finished up 11 and down only twice. In February, the ratio falls to 9:5. But whether this year proves an exception to the summer rule is up for debate. As investors await the outcome of the Sino-British talks and continue to feel the perceived threat of China's MFN status, the summer months may show a lull, some analysts suggest.