Forty-seven piglets were inoculated with doses of 100 to 50,000 sporulated oocysts of Isospora suis. After 5-7 days oocysts were found in faeces. The patent period extended from 8 to 16 days. The shedding of oocysts showed a cyclic pattern with 2-3 peaks separated by intervals of approximately 5 days. Subpatent periods were often seen between the peaks. The level of oocyst shedding during the initial days of the patent period reflected, to some extent, the inoculation dose. However, a maximum of OPG at the 100,000 level was observed among one or more piglets from all groups, regardless of the inoculation dose. Among the majority of piglets inoculated with more than 100 oocysts, the highest OPG-figures were observed in the first peak of the cyclic pattern. Unlike this, the maximum of OPG was observed in the second peak of the cycle among 6 of the 7 piglets inoculated with 100 oocytes only. The triphasic pattern was most pronounced in the low dose group. The marked upscaling of oocyst production, as particularly registered in the low dosed groups, seams to explain at least part of the problems met under practical conditions, when trying to eliminate the transmission of oocytes between successive litters in the farrowing boxes. The cyclic excretion pattern and an apparent absence of autoinfection may indicate that the development of I. suis in the host includes several oocyte producing generations descending from the same initial infection. The presence of subpatent periods can probably explain the marked variation in OPG, as they are often recorded when examining faecal samples from piglets, even when the samples are originating from the same litter.