Uteroglobin is a protein secreted by the rabbit uterus in response to progesterone. In cell-free translation systems, the mRNA for uteroglobin codes for a protein larger in size than the secreted protein. To investigate the relationship between these two forms, the NH2-terminal sequences of pre-uteroglobin and of uteroglobin have been determined. Uteroglobin was purified from rabbit uterine flushings and pre-uteroglobin was obtained by immunoprecipitation of the products of translation of poly(A)-rich endometrial RNA in the wheat germ system in the presence of single or multiple radioactive amino acids. Sequencing was performed by automated Edman degradation and the residue at each cycle was identified by chromatography. The larger size of pre-uteroglobin is accounted for by a 21-amino-acid leader sequence, containing 15 hydrophobic residues, at the NH2 terminus. The sequence Thr-Leu-Ala-Leu occurs twice in the leader region. In common with other secretory proteins, the transient hydrophobic extension at the NH2 terminus of pre-uteroglobin may function to assist transfer of the nascent peptide into the lumen of the endoplasmic reticulum, as proposed in the "signal" hypothesis.