Canadian stock prices took their worst beating in almost 50 years during the first quarter of this year as trading value slumped dramatically, according to figures released by the Toronto Stock Exchange.

The Toronto exchange is Canada's largest and accounts for the lion's share of Canadian stock trading activity.

The composite index of 300 stocks, the main indicator of Toronto price levels, fell 18.75 percent during the past three months, reducing the value of Canadian shares listed on the exchange by $23.8 billion. The exchange estimates the current value of listed shares at $103.2 billion.

The big slump is a continuation of a downturn that began in July 1981. During the past 12 months, the index is down 31.94 percent, the fourth worst loss for any 12 month period since 1934.

Exchange figures show that March trading value on the Toronto, Montreal, Vancouver and Alberta exchanges fell 50.4 percent to $1.76 billion from $3.55 billion in March, 1981. Volume dropped 30 percent to 225.5 million shares from 322.1 million a year ago.

The Toronto market reported a 46.1 percent drop in value at $1.38 billion, down from $2.57 billion a year earlier. Volume sank 19 percent to 114 million shares from 140.8 million in the 1981 month.

Value declines on other exchanges were larger with Montreal showing a 47.1 percent drop, Vancouver a 78.9 percent decline and Alberta a 78.1 percent reduction in value. The Vancouver and Alberta exchanges deal mainly in speculative resource stocks.

Volume declines in the latest month were 23.1 percent for Montreal, 41.4 percent for Vancouver and 37.1 percent for Alberta. For the latest three months, Canadian trading value dropped 46.8 percent to $4.68 billion from $8.8 billion in the 1981 period. Volume sank 33.8 percent to 609.2 million shares from 920.2 million a year ago.

Toronto recorded a 42.8 percent decline in the first quarter value at $3.66 billion, compared with $6.39 billion in the 1981 quarter. Volume in the three months was down 22.9 percent at 294.5 million shares from 382.2 million shares a year ago.

Canadian brokers have been trying to reduce staff size through attrition and staff layoffs of about 10 percent are common in the industry as a result of the decline in activity.

Some firms have asked employes, partners and senior officers to take pay cuts of about 10 percent.