Boron (B) enters cells as the uncharged boric acid, a small neutral molecule with sufficient lipid solubility to cross cell membranes without the aid of transport proteins. The extent to which the observed uptake rates for B in plants can be explained by this simple physical process was examined by applying treatments expected to inhibit the membrane transporters most likely to be involved in B transport. These experiments established that at least 50% of B uptake could be facilitated by transporters. The B transport characteristics of two barley aquaglyceroporins, HvPIP1;3 and HvPIP1;4, were investigated using yeast complementation assays. Expression of both genes in yeast resulted in increased B sensitivity. Transport assays in yeast confirmed that HvPIP1;3 and HvPIP1;4 are both capable of transporting B. The physiological role of these HvPIP1 genes in B transport is uncertain since their expression was not responsive to B nutritional status, and they continued to be expressed under toxicity conditions.