A large number of myostimulatory neuropeptides from neurohaemal organs of the American cockroach have been described since 1989. These peptides, isolated from the retrocerebral complex and abdominal perisympathetic organs, are thought to be released as hormones. To study the coordinated action of these neuropeptides in the regulation of visceral muscle activity, it might be necessary to include myoinhibitors as well, however, not a single myoinhibitory neuropeptide of the American cockroach has been described so far. To fill this gap, we describe the isolation of LMS (leucomyosuppressin) and Pea-MIP (myoinhibitory peptide) from neurohaemal organs of the American cockroach. LMS was very effective in inhibiting phasic activity of all visceral muscles tested. It was found in the corpora cardiaca of different species of cockroaches, as well as in related insect groups, including mantids and termites. Pea-MIP which is strongly accumulated in the corpora cardiaca was not detected with a muscle bioassay system but when searching for tryptophane-containing peptides using a diode-array detector. This peptide caused only a moderate inhibition in visceral muscle assays. The distribution of Pea-MIP in neurohaemal organs and cells supplying these organs with Pea-MIP immunoreactive material, is described. Additionally to LMS and Pea-MIP, a member of the allatostatin peptide family, known to exhibit inhibitory properties in other insects, was tested in visceral muscle assays. This allatostatin was highly effective in inhibiting spontaneous activity of the foregut, but not of other tested visceral muscles of the American cockroach.