The fine structure of myometrium from pigs collected at well-defined stages during pregnancy was investigated by transmission electron microscopy. The morphology of the pregnant myometrium resembled that in non-pregnant pigs. Thick myofilaments were conspicuous during early pregnancy, inapparent during mid-pregnancy and visible again by days 80-84 and towards parturition. Gap junctions were extremely rare throughout pregnancy. Only occasionally were nerve axons observed.