Several in vitro studies reported on increased levels of progesterone secretion from placental tissue. This was not tested in vivo. To study the issue we examined progesterone levels in women presenting with premature labor before treatment, and 24 and 48 h after beginning ritodrin treatment. A control group consisted of women treated with indomethacin for preterm labor. There were 13 women in the ritodrin group and 12 in the control group. The mean concentration of progesterone in the ritodrin group was 161.7 ± 74.2, 132 ± 50.2 and 145 ± 49.8 ng/ml before treatment, and 24 and 48 h after ritodrin treatment, respectively. The mean progesterone concentration of the group treated with indomethacin was 129 ± 45.6, 132 ± 49.5 and 138.6 ± 53.5 ng/ml before treatment, and 24 and 48 h after treatment, respectively. There were no statistically significant differences between the groups or within each group before and after treatment. Ritodrin treatment does not affect progesterone levels in vivo as demonstrated by in vitro studies.