The records of the Philadelphia Zoological Garden suggest that social (intraspecific) interaction may be a major factor in the origin and progress of stenosing lesions of the coronary arteries that lead to myocardial infarction in many species of mammals and birds. 1 " 4 The present study was designed to explore this possibility. The origin and progress of coronary arterial lesions in cocks and hens have been traced from age 7 to 51 weeks, and rates of progress of the lesions measured

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... vely and related to age, sex, and social situation. Methods Chickens from a closed flock of pullorum-free, single-comb, white leghorns of the Hy-line strain were hatched in one lot and brooded as one flock through 7 weeks of age then assigned randomly to cages in one large room with 15 hours of light per day. Ten each of cocks and hens were killed as a sample of brooder stock at 7 weeks of age. The birds were vaccinated and their beaks trimmed at appropriate intervals. Commercial poultry rations were fed ad libitum and water from a deep well was supplied through an automatic system. Cocks weighed 2 to 3 kg and hens 1 to 2 kg at sexual maturity: about 21 weeks of age. Two-tiered commercial battery cages divided by wooden partitions allowed 2 square ft per bird in all situations except one in which 4 groups of 12 (6 cocks-6 hens) were allowed 8 square ft per bird. The chickens were studied in the following numbers and situations: a) 40 each of cocks and hens caged separately, b) 64 cock-hen pairs, c)