Dow Chemical Co.'s 1981 profits dropped by 30 percent to $563.5 million ($3 a share) from $804.5 million ($4.42) the previous year, although sales increased 11.7 percent to $11.87 billion from $10.63 billion.

Dow President Paul F. Oreffice blamed the reduced profits on "severely depressed" economic conditions worldwide. "This has now become the most serious global recession in many years for our industry," Oreffice said. "Our earnings in the fourth quarter suffered from the combined effect of reduced operating rates and lower prices."

Fourth-quarter profits were $66.5 million (35 cents) on sales of $2.99 billion compared with profits of $242.3 million ($1.33) on sales of $2.77 billion in the last three months of 1980.

Dow noted that its earnings were affected by a change in the company's method of accounting for inventories, which increased profits by $101 million (54 cents) for the year.

Citing improvement in nearly all its businesses, PepsiCo Inc. yesterday reported profit increases of 51.3 percent for the fourth quarter and 21.8 percent for all of 1981.

The company said it earned $91.4 million (99 cents a share) in the three months ended Dec. 26 compared with $60.4 million (67 cents) a year earlier. The 1980 results included a $17.8 million write-off of operating rights for the company's trucking subsidiary.

Revenue in the final 1981 quarter climbed 14.8 percent to $2.17 billion from $1.89 billion.

Full-year profits rose to $333.5 million ($3.61) from $273.9 million ($3.01) as revenues grew 17.8 percent to $7.03 billion from $5.98 billion in 1980.