iitc the well-known and striking efiects of protein cleprivatioii on growth, relatively few liistologic studies on skeletal growth aiicl ciiclocho~iclral ossification during protein cleficieiicy in the rat have been carried out. Lussier ('51) has i*ecently i.eportecl a stud>-011 the cffccts in adult rats of iiiaiiiteiiaiice 011 a protein-free diet for 30 days, and Chawla aiid Glickmaii ( '51) on thc effects of a 2% protein diet in \wanling rats f o r 8 weeks. The cfYccts of a diet dcficicnt in a single essciitial aniirio acid, lysinc, on skeletal growth iii young rats ( H a r r i s ct al., '43) arid in adult rats (Gillespie et al., '43) and the effects of similar diets deficient in pheiiylalaiiine, leuciiic, o r histidine (Mauii ct a]., '45a, '431, '46 ; Schwartz et al., '51) had previously been reported. The rcsults of a deficieiiey of protein or of a single essential amino acid on endochoiidral ossification of the long bones as described in the above studies inap bc summarized as follows :Aided by grants from the Smerican Foundation f o r Dental Science, the Department of Agriculture of the TJniversity of California, tlic Roclie A i m i~ crsary Foluldation, arid the Cn1ifolni:L State Dental Association. The following nlaterials IT ere generously contributed : crystalline R vitamins and ;ilplia-tocopherol from JIerck and Comp:~iiy, Iiic*., Ralrwap, X e w Jersep ; liver eluate powder (lab. no. ;-,j293) from Ledc~rlc T,abor:ttories, Tnc., Pearl River, Nrw Pork. * Mazola.