Summary One- and three-month-old male and female rats were injected intraperito-neally with d-tubocurarine in various doses. These animals were then placed on their sides and their ability to regain the upright position determined. No difference in sensitivity to d-tubocurarine was found to exist between one-month-old females, one-month-old males, and 3-month-old males. The 3-month-old females, however, were significantly more sensitive than any of the other groups. Gonadec-tomy performed at age one week failed to abolish the sex difference in curare sensitivity in animals tested at 3 months of age. We would like to express our appreciation to Dr. A. Varma for statistical analysis of the data, and to Drs. H. Houston Merritt, and L. P. Rowland for review of the manuscript. We would also like to thank Mrs. Arleene Ozerkis, Mr. Robert Rodvien, Mr. Robert Lattes, and Mrs. Barbara Wolf for technical help. We are grateful to E. R. Squibb for supplies of Intocostrin.