Nineteen gilts were used in an experiment to examine the relationship between rate of development and embryonic sex on day 10 of pregnancy. All gilts were mated to the same boar approximately 24 h after detection of second oestrus. They were individually housed and fed similar diets until slaughter on day 10 of gestation (day 0 = day of insemination) for subsequent recovery of the conceptus. All conceptuses were photographed and their surface areas (mm2) measured by tracing outlines on a digitized tablet interfaced with a computer program. Within each litter, individuals were categorized as small, medium or large by three equal divisions of the size range between the smallest and largest member. Conceptuses were individually cultured in Medium 199 with 1% colcemid and stained with 4% Giemsa. Metaphase spreads were located and sex was determined by presence or absence of the Y chromosome in at least two spreads from each specimen. A total of 214 conceptuses were recovered but only 125 (58%) were successfully karyotyped. The overall sex ratio was not significantly different from 1:1 (57 males and 68 females; P > 0.25). Sex was determined in 51 of 88 small embryos, 22 of 44 medium embryos and 52 of 82 large embryos and males represented 9 (17.6%), 10 (45.5%) and 38 (73%), respectively. Logistic analysis indicated significantly more females in the small and significantly more males in the large groups (P < 0.001). Results demonstrate that most male conceptuses grow faster than females before commencement of attachment to the uterine lining.