Six-year-old Consolidated Rail Corp. had its first profitable year in 1981, it reported yesterday.

The federally financed freight railroad also reported a profit in the fourth quarter of $26.4 million on consolidated revenues of $1 billion compared with a loss of $12 million on revenues of $1.04 billion in the same quarter of 1980.

For the year, Conrail reported profits of $39.2 million on revenues of $4.2 billion compared with a loss of $243.7 million on revenues of $3.98 billion in 1980.

The results were reported under generally accepted accounting principles, which publicly owned railroads also use to report profits to the investment community. Under the financial reporting requirements the Interstate Commerce Commission imposes on railroads, Conrail reported a loss of $4.4 million for last year's fourth quarter compared with a loss of $45.4 million for the same period of 1980. For the year as a whole, under ICC reporting, there is a loss of $106.6 million compared with a loss of $405.8 million in 1980.

The fourth quarter was the third successive quarter for which Conrail has reported net income. Conrail, the successor to bankrupt railroads in the Northeast and Midwest, has not required federal funding since June 1981.

Aetna Life & Casualty Co., a leading insurance concern with interests in real estate and communications, reported yesterday that profits fell 25 percent in the final three months of 1981 and slid 18 percent for the year.

The declines came despite improved operating earnings for the fourth quarter. Investment losses contributed to the earnings decline, the company said. Aetna is the country's second-largest diversified financial company.

Aetna reported capital losses of $36.6 million in the fourth quarter compared with capital gains of $12.8 million in the same period of 1980. For all of 1981, capital losses totaled $28.9 million, mostly as the result of the sale of low-yield bonds, compared with gains of $53.3 million in 1980, reflecting the sale of two office buildings and Aetna Business Credit Inc.

For the fourth quarter of 1981, Aetna earned $109.9 million ($1.37 a share) on revenue of $3.55 billion, compared with earnings of $146.6 million ($1.82) on revenue of $3.29 billion in the final three months of 1980.

Earnings for the fourth quarter and all of 1981 included a $30 million after-tax gain from the sale of an interest in property in Denver.

For all of 1981, Aetna earned $462.2 million ($5.75) on revenue of $13.53 billion, compared with earnings of $561.6 million ($6.96) on revenue of $13.32 billion in 1980.

Uniroyal Inc., citing the recession and the depressed auto industry, said yesterday that its fourth-quarter profits fell to $300,000 (a loss of 3 cents a share) from $4.7 million (17 cents a share) a year earlier. Sales fell 6 percent to $558.4 million from $596.2 million.

For the full year 1981, Uniroyal's profit rose to $51.6 million ($1.76) against a loss of $7.8 million in 1980. However, annual sales fell 2 percent to $2.26 billion from $2.3 billion.