During early pea fruit growth, the physiological roles of 4-chloroindole-3-acetic acid (4-Cl-IAA) and IAA, natural pea auxins, in regulating gibberellin (GA) 20-oxidase gene expression (PsGA20ox1) were tested with 4-position, ring-substituted auxins that have a range of biological activities (fruit growth). The effect of seeds, and natural and synthetic auxins (4-Cl-IAA, and IAA; 4-Me-IAA, 4-Et-IAA and 4-F-IAA, respectively), and auxin concentration (4-Cl-IAA) on PsGA20ox1 mRNA levels in pea pericarp were investigated over a 24 h treatment period. The ability of the 4-substituted auxins to increase PsGA20ox1 mRNA levels in deseeded pericarp was correlated with their ability to stimulate pericarp growth. The greatest increase in pericarp PsGA20ox1 mRNA levels and growth was observed when deseeded pericarps were treated with the naturally occurring pea auxin, 4-Cl-IAA; however, IAA was not effective. Silver thiosulfate, an ethylene action antagonist, did not reverse IAA's lack of stimulation of PsGA20ox1 over the control treatment. 4-Me-IAA was the second most active auxin in stimulating PsGA20ox1 and was the second most biologically active auxin. Application of the 4-substituted IAA analogs, 4-Et-IAA and 4-F-IAA, to deseeded pericarps resulted in minimal or no increase in PsGA20ox1 transcript levels or pericarp growth. Pericarp PsGA20ox1 mRNA levels increased with increasing 4-Cl-IAA concentration and showed transitory increases at low 4-Cl-IAA treatments (30 to 300 pmol). These results support a unique physiological role for the auxin 4-Cl-IAA in the regulation of GA metabolism by effecting PsGA20ox1 expression during early pea fruit growth.