Abstract Earlier work from our laboratory on Indian mustard (Brassica juncea L.) identified the following rate-limiting steps for the assimilation and volatilization of selenate to dimethyl selenide (DMSe): (a) uptake of selenate, (b) activation of selenate by ATP sulfurylase, and (b) conversion of selenomethionine (SeMet) to DMSe. The present study showed that shoots of selenate-treated plants accumulated very low concentrations of dimethylselenoniopropionate (DMSeP). Selenonium compounds such as DMSeP are the most likely precursors of DMSe. DMSeP-supplied plants volatilized Se at a rate 113 times higher than that measured from plants supplied with selenate, 38 times higher than from selenite, and six times higher than from SeMet. The conversion of SeMet to selenonium compounds such as DMSeP is likely to be rate-limiting for DMSe production, but not the formation of DMSe from DMSeP because DMSeP was the rate of Se volatilization from faster than from SeMet and SeMet (but no DMSeP) accumulated in selenite- or SeMet-supplied wild-type plants and in selenate-supplied ATP-sulfurylase transgenic plants. DMSeP-supplied plants absorbed the most Se from the external medium compared with plants supplied with SeMet, selenate, or selenite; they also accumulated more Se in shoots than in roots as an unknown organic compound resembling a mixture of DMSeP and selenocysteine.