Sales at Best Products Co., the nation's largest catalogue showroom retailer, climbed significantly, but the company got caught between heavy advertising and expensive expansion in the first quarter and ended up losing money.

Best lost $3.2 million in the quarter ended May 1, compared with losses of $300,000 in the spring of 1981. That means the reporting period, traditionally a weak time of year for Best, was 10 times worse than last year.

Sales picked up 13 percent from this time last year, bringing in $215.8 million against $191.7 million, largely because of an aggressive direct mail and newspaper advertising campaign, company officials said yesterday. But the advertising costs cut deeply into those gains.

Best opened eight new showrooms during the quarter, which company officials said was a drain on profits. Because of those openings, officials said, operating expenses increased as a percentage of sales, as did the quarter-ending inventory.

Best officials wouldn't speculate yesterday on the summer quarter, but they did say they intended to keep up their ad campaigns.

And Best said it wouldn't alter its expansion pace, with three new showrooms scheduled to open this fall. Right now, the firm has 108 stores in 11 states.