To develop a cytotoxic agent selective for pituitary corticotropes, an analog of CRF, [Nle^21,38, Arg³⁶] rat(r) CRF, was conjugated to the plant toxin gelonin using glutaraldehyde as a cross-linking agent. In a two-step synthesis, glutaraldehyde was first coupled to the amino-terminal of the CRF analog. This was then reacted with gelonin to form a product, which contained 20 mol [Nle^21,38, Arg³⁶]rCRF/mol gelonin, as indicated by amino acid analysis. Efficacy and specificity of the gelonin- [Nle^21,38, Arg³⁶]rCRF conjugate were assessed in cultured rat anterior pituitary cells 3 days after a 12-h exposure to the cytotoxic conjugate, CRF, or gelonin. While pretreatment with the cytotoxic gelonin-[Nle^21,38, Arg³⁶]rCRF conjugate decreased cellular ACTH content by 74%, pretreatment with appropriate concentrations of CRF had no significant effect on cellular ACTH content. The ACTH secretory response to 2.5 nM CRF was completely eliminated by pretreatment with the gelonin- [Nle^21,38, Arg³⁶]rCRF conjugate; in contrast, pretreatment with identical doses of CRF had minimal effect. Neither pretreatment with CRF nor pretreatment with the gelonin- [Nle^21,38, Arg³⁶] rCRF conjugate had any effect on the content of LH or the LH secretory response to GnRH. These results indicate the efficacy and specificity of the conjugate of [Nle^21,38, Arg³⁶]rCRF to gelonin in eliminating CRF target cells. (Endocrinobgy121:1454–1460, 1987)