[Show abstract][Hide abstract] ABSTRACT: Progesterone (P4) and nitric oxide (NO) suppress uterine contractility (CTX). This study compares the effects of P4 to sodium nitroprusside (SNP, an NO donor) and their combination on human CTX of term/preterm and labor/nonlabor tissues. Uterine tissues (n = 128) from women (n = 28) undergoing Cesarean were suspended in organ baths. Tissues (n ≥ 6/group) were treated with vehicle, P4, SNP, or combinations. A subset of tissues (n ≥ 2/group) from term/preterm ± labor and nonpregnant patients was analyzed with P4 alone. Analysis of variance (ANOVA) was used for statistical differences (P < .05). The combination of P4 with SNP significantly suppresses CTX (% inhibition of -127.1 ± 14.5) to the levels lower than with either P4 (-20.1 ± 8.6) or SNP alone (-72.0 ± 11.2). Suppression of P4 is similar in term, preterm, and nonpregnant tissues, with increased sensitivity in laboring tissues. This indicates that P4 or SNP alone may be used for preterm labor and their combination may be more successful.