Abstract Alkaline ribonuclease activity in 257 samples of sera obtained from 64 pregnant women in 19th to 42nd week of pregnancy was determined by a simple method. Pregnant women showed a significantly higher (15%, P < 0.02) activity than nonpregnant women of reproductive age. There is a gradual and significant increase in activity from weeks 10–20 to 21–30 and then to 31–42 of pregnancy. The pattern of enzyme level during weeks 10–42 of pregnancy does not show a continuous and regular increase as shown by enzymes and hormones of exclusively placental origin but is punctuated with many irregular rises and falls. Enzyme activities in a group of women showing complicated pregnancies were determined and compared with those found in normal pregnant women.