Development of porcine conceptuses included transitions of five stages: blastocysts of spherical, ovoid, and tubular forms containing an embryonic disc and trophoblast, extremely elongated filamentous blastocysts, and stages of embryogenesis between days 9 and 18 after mating. Embryonic survival was reduced by 17% during this period. In this litter-bearing species, intense alteration in distribution patterns occurred at days 11 and 12, when blastocysts rapidly elongated to filamentous forms. Increased embryo mortality did not result from rapid changes in distribution patterns of conceptuses within the same uterine horn at this time. Filamentous blastocysts quickly reached lengths often exceeding 60 cm, and these conceptuses became regularly spaced with no overlap of tubular membranes from other embryos in that horn. The number of concep-tuses within a uterine horn ranged from 1 to 13. Protein in individual concep-tuses was used as an indicator of growth and denoted exponential increase, but at a lower rate for blastocysts of spherical, ovoid, and tubular forms as compared with that found in filamentous blastocysts and embryogenesis stage conceptuses. Growth of a conceptus, based upon its protein content, was independent of the developmental stage or potential loss of those neighbors nearest that conceptus.