In order to localize the site of production of nitrogen monoxide (NO) in first trimester human pregnancy, the cytosol and microsome fractions prepared from homogenized primordial placentas were tested for NO-synthase (NOS) activities by measuring the NADPH-dependent conversion of [3H]arginine to [3H]citrulline. Our results demonstrate that Ca(2+)-dependent enzyme activities are present in both fractions, whereas microsomes exhibit significant Ca(2+)-independent enzyme activity too. The highest specific activity is measurable in the presence of Ca2+ with microsomes, this activity is about 2-fold higher than the Ca(2+)-dependent specific activity of the cytosol. The Ca(2+)-independent specific NOS activity is about 30% of the Ca(2+)-dependent microsomal activity. The microsomal Ca(2+)-dependent NOS activity is inhibited by 50% in the presence of 0.5 mM aminoguanidine (AG), whereas the Ca(2+)-independent activity does not respond to this concentration of AG, suggesting that it is not the inducible isoform of NOS. Our results indicate that (I) NOS activity is present from an early phase of placental development, (II) the NOS activity is of trophoblastic origin, since the primordial placenta is avascular and (III) NO-production by the primordial placenta can proceed in the absence of any Ca(2+)-mobilizing agonist.