Investigations on mechanisms of metastatic tumour spread revealed a role for compounds that inhibit tumour dissemination at the time of hematogenous dissemination. The platelet aggregation inhibitor prostacyclin and its stable analogues were shown to inhibit tumour-cell-induced platelet interaction as well as tumour cell adhesive mechanisms. This study concentrates on the effect of the stable prostacyclin analogue cicaprost: 5-[(E)-(1S,5S,6S,7R)-7-hydroxy-6-[(3S,4S)-3-hydroxy-4-methylnona-1 ,6- diinyl]-bicyclo[3,3,0]octan-3-ylidene]-3-oxapentanoic acid (Schering AG), as cyclodextrin clathrate, on spontaneous tumour metastases of two different carcinomas of the rat. In Cop rats bearing spontaneously metastasizing R 3327 MAT Lu prostate carcinomas, cicaprost (1.0 mg/kg p.o. daily) inhibited the number of lung metastases by about 80%, whereas the lower doses (0.1 and 0.5 mg/kg) exhibited borderline efficacy. In female Wistar-Furth rats bearing s.c. implanted SMT 2A mammary carcinomas, spontaneously metastasizing into regional lymph nodes and lungs, cicaprost (0.1, 0.5 and 1 mg/kg) p.o. daily exhibited a dose-dependent inhibition of the number of lung metastases. Five out of ten animals treated by 1 mg/kg were free of visible lung metastases. The weight of the axillary lymph node was significantly reduced by the 1 mg/kg dose of cicaprost, whereas lower doses had no effect on the weight of the lymph nodes. The growth of the primary tumour was not influenced by cicaprost in the R 3327 MAT Lu prostate carcinoma nor in the SMT 2A mammary carcinoma in the dose range tested. In conclusion, the stable prostacyclin analogue cicaprost exhibits a strong antimetastatic action in two metastasizing tumours of the rat and interferes with the steps not only of haematogenous, but also of lymphogenous metastasis.