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Summary The mechanism by which exogenous auxins inhibit embryo development from embryogenic callus is not known. In sweet potato [ Ipomoea batatas, (L.) Lam.] we investigated the hypothesis that embryo development was subjected to endogenous IAA transport by studying embryogenesis from calli in response to various concentrations of either 2,4-dichlorophenoxyacetic acid (2,4-D), 2,3,5-triiodobenzoic acid (TIBA), 7-aza-indole (AZI) or p-chlorophenoxyisobutyric acid (PCIB). Additions of AZI and PCIB at 1 to 5 μM did not affect morphogenesis. Additions of TIBA and 2,4-D at 5 μM inhibited embryo formation and promoted embryogenic callus growth. Embryo development was restricted to increasingly earlier stages, i.e. from the torpedo stage toward the pro-embryo stage, as TIBA was gradually increased from 0 to 6μM, or as 2,4-D was gradually increased from 0 to 5μ/tM. TIBA specifically inhibits polar IAA transport, thus, our results suggest that embryo development may coincide with endogenous IAA transport and that, inhibition of embryogenesis from embryogenic calli by exogenously supplied auxins may be a consequence of the disruption by exogenous auxins of endogenous IAA efflux from embryogenic loci.