Pfizer Inc. said Thursday the strength of the dollar helped depress its net income by 5.9 percent in the fourth quarter of 1989 despite a 9.2 percent increase in sales. For the three months ended Dec. 31, the pharmaceutical company's net income fell to $133.3 million, or 79 cents a share, from $141.6 million, or 84 cents a share, a year earlier. Fourth-quarter sales rose to $1.49 billion from $1.37 billion. The dollar's strength against foreign currencies, increased marketing expenses for new products and continued high research spending contributed to an 8.1 percent decline in net income for all of 1989, even though sales increased 5.3 percent, Pfizer said. ``Although these investment expenses will again be high in 1990, the sales momentum of our new products is increasing,'' Chairman Edmund T. Pratt Jr. said in a statement. Pratt said a new extended release version of Pfizer's Procardia prescription drug for angina and hypertension tallied $40 million in sales in just three months on the U.S. market. The company expects to win U.S. regulatory approval this year to sell another angina and hypertension drug, Norvasc, which it currently sells in Britain and Ireland, he said. The dollar's strength reduced sales by two percentage points and turned what would have been a modest growth in profits into a decline, the company said. A strong dollar makes U.S. products more expensive, and therefore less attractive, overseas. For the full year, net income fell to $727.3 million, or $4.31 a share, from $791.3 million, or $4.70 a share. Full-year sales rose to $5.67 billion from $5.39 billion. If the dollar hadn't risen in value last year, sales would have been three percentage points higher and the profit decline would have been cut in half, Pfizer said. The company's sales of pharmaceuticals increased 11 percent in the latest year while sales of hospital products rose 3 percent. Consumer products sales rose 12 percent, specialty chemicals sales gained 6 percent and sales of animal health care products were up 9 percent.