1. Treatment of the chick embryo with the lathyrus factor, β-aminopropionitrile, produces inhibition of polymerization of newly synthesized collagen at the interand intra-molecular level (Levene & Gross, 1959; Martin, Gross, Piez & Lewis, 1961); this is manifest as an increase in the fragility and solubility of collagen from the connective tissues. 2. This treatment simultaneously produced increased hydration of the tissues, particularly noticeable in the swollen long-bone cartilages. 3. This increased hydration of cartilage was found to result from treatment with any of the known lathyrogenic compounds, but not with any of the structurally related but non-lathyrogenic analogues. 4. The hydration effect paralleled the collagen solubility effect, both being dosage-dependent at the lower levels. 5. The hydration effect could not be reversed by treatment in vivo with either pyridoxal or glyceraldehyde, or by Benadryl, an antihistamine that inhibits cell water movement. 6. The possible causes of this effect have been considered in the light of the known properties of cartilage.