1. There is conflicting evidence in the literature concerning the role of cyclic GMP in the regulation of myometrial contractility and the importance of hormonal status on the uterine response to cyclic GMP-elevating agents. The objective of the present study was to investigate further the importance of cyclic GMP in the control of uterine contractility, by monitoring the effects of cyclic GMP-elevating agents on spontaneous contractions and cyclic GMP levels in myometrial strips from pregnant rats and from ovariectomized rats under the influence of oestrogen and/or progesterone. 2. Sodium nitroprusside (SNP) 5 mM, atrial natriuretic peptide (ANP) 100 nM, L-arginine 1 mM and 8-bromo-cyclic GMP 100 mM had no relaxant effect on the spontaneous contractions of myometria from pregnant rats or from ovariectomized rats under the influence of oestrogen or progesterone. 3. Tissue levels of cyclic GMP were significantly elevated by SNP in all treatment groups, including pregnant animals. For example, in ovariectomized, progesterone-treated rats, SNP raised cyclic GMP levels approximately 8 fold from a basal level of 2.9 +/- 0.4 pmol mg(-1) protein to 24.8 +/- 4.0 pmol mg(-1) protein. ANP increased cyclic GMP levels approximately 2 fold in all treatment groups, except in the pregnant animals. L-Arginine elevated cyclic GMP significantly only in ovariectomized, vehicle-treated myometria. 4. The activity of cyclic GMP-dependent protein kinase (PKG) was significantly increased (3 fold) in myometria exposed to SNP (5 mM). Thus, the inability of SNP to relax uterine preparations was not due to a failure of SNP-elevated cyclic GMP to activate PKG. 5. The more potent NO donor, S-nitroso-N-acetylpenicillamine (SNAP), at a concentration of 100 microM was able to inhibit spontaneous contractions significantly in myometrial preparations from both non-ovariectomized and ovariectomized rats treated with oestrogen or progesterone. 6. Tissue levels of cyclic GMP were markedly increased by SNAP at concentrations of 10, 30 and 100 microM. At 100 microM, cyclic GMP levels increased from 1.9 +/- 0.2 pmol mg(-1) protein to 74.0 +/- 18.0 pmol mg(-1) protein. However, complete or partial blockade of SNAP-induced increases in cyclic GMP levels by the soluble guanylyl cyclase inhibitor, ODQ (25 microM), had no effect on the relaxant response to SNAP. Thus, the relaxant effect of SNAP in this tissue appears to be mediated via a mechanism independent of cyclic GMP. 7. Taken as a whole, the results of the present study indicate that cyclic GMP does not play an important role in the control of contractility in the rat uterus.