Thousands of years ago, the Book of Leviticus declared that a woman was worth only three-fifths as much as a man: "Your valuation of a male . . . shall be 50 shekels of silver. . . . If the person is a female, your valuation shall be 30 shekels."

Until recently, amazingly enough, Labor Department statistics showed that the annual earnings of year-round full-time female workers seemed permanently pegged at three-fifths of the level for male workers in similar jobs.

In 1939, median earnings of such women were 58 percent those of men, and in 1979 they were 60 percent. Since then, the ratio has been going up, and it reached about 65 percent in 1987.

A new Labor Department study confirms that there has been an improvement in women's pay in relation to men's, and the new figure is not just a statistical glitch caused by increases in hours worked by year-round women employees.

"The female-male earnings gap narrowed significantly between 1979 and 1987, reflecting increases in earnings per hour rather than in hours worked," the study in the department's Monthly Labor Review says.

Economists have been aware of the earnings-ratio improvement for several years, but some suggested the change did not reflect a narrowing of the pay gap but simply changes in hours worked.

This fear arose because under the Bureau of Labor Statistics (BLS) definition used to determine earnings ratios between men and women, someone who worked part time as many as 25 weeks a year and full time the rest was considered a year-round full-time worker. This led to the possibility that the increase in women's annual wages was the result of women working more hours so that more of the 25 part-time weeks became full-time weeks.

But the authors of the study, BLS economists Michael W. Horrigan and James P. Markey, say that is not the case.

Their study used a new definition eliminating from the male-female comparison people who worked part time, and identifying a person as a "true year-round full-time worker" only if he or she worked at least 35 hours a week for 50 weeks or more per year.

They found that if the comparison was restricted to workers covered by the new definition, the ratio of women's wages to those of men still worked out to 60 percent in 1979 and rose to 65 percent in 1987. Using this definition, median annual earnings of "true year-round full-time" women workers in 1987 were $17,531, or 65 percent of the $26,959 median for men. From this they concluded that the earnings improvement was based on a narrowing of the gap in pay rates, not changes in hours.

"Traditional lower pay rates and earnings for women were based in part on the fact that, in the past, women tended to receive less education and training than men, and often in less rewarding skills -- less math, less science, less professional training," said Heidi Hartmann, director of the Washington-based Institute for Women's Policy Research.

"Another factor," Hartmann said, "was the tendency of women to be segregated in traditional 'women's occupations' such as teaching, clerical work and housecleaning where pay was lower, and often shut out of more remunerative lines of work such as medicine, law and business management."

Added to that, she said, were work interruptions for childbearing and other family needs, so that women worked fewer years and had more breaks in service than men, earning less seniority and fewer promotions.

She also cited more unionization in some types of male-dominated work -- manufacturing, for example -- and pay discrimination against women, which many researchers say persists despite the 1963 Equal Pay Act.

Hartmann and Markey said labor force specialists believe many of these causes of real pay differentials are changing -- women are getting broader education, reaching out to higher-paying fields formerly reserved to men, working more years with fewer interruptions. "Younger women are entering the better occupations now," Hartmann said. "More and more women are working and staying in."

Men's wages are also stagnating, allowing women to catch up, Hartmann and Markey noted.

Hartmann said she believes the gap between the annual earnings of year-round, full-time male and female workers will continue to close, although, she pointed out, "It is is easier to close the gap when times are good, and the economy is not so good now."