The possibility of a periparturient rise (PPR) in the excretion of Cryptosporidium parvum oocysts and Giardia duodenalis-type cysts and its implications in the epidemiology of these 2 pathogens were investigated. Fecal samples were taken from 32 lambing ewes every 2 wk from 4 wk before lambing until 8 wk after the start of lambing. Samples from both ewes and lambs were examined with a quantitative immunofluorescence assay. Excretion of Giardia sp. cysts by ewes increased at 2 wk prepartum, peaked at zero and 4 wk postpartum, and returned to low levels at 6 and 8 wk postpartum. Some ewes also excreted C. parvum oocysts at the start of lambing. Excretion rates and intensities of Giardia sp. cysts by lambs were high, whereas those of C. parvum oocyst excretion were light. The PPR in excretion of Giardia sp. cysts by ewes was considered to be the major source of giardiasis for lambs. Excretion of C. parvum oocysts by ewes at parturition also played a role in initiating cryptosporidiosis in lambs.