Experiments were undertaken to test how aquaporins (AQPs) facilitate the uptake of water by roots of Pisum sativum. Changes in PsPIP2-1 gene expression and root hydraulic conductivity (Lpr) were measured in response to the time of day as well as treatment of the roots with a compound that reduced Lpr [i.e. mercuric chloride (HgCl2)] and one that was intended to increase Lpr [abscisic acid (ABA)]. There was a diurnal rhythm in PsPIP2-1 expression in lateral roots that was strongly correlated with diurnal changes in Lpr. Taproots also displayed a rhythm in PsPIP2-1 expression, but this was offset from that of Lpr. This suggested that changes in Lpr were mediated by changes in PsPIP2-1 mRNA transcript abundance. Reduction of Lpr by HgCl2 treatment was accompanied by an increase in PsPIP2-1 expression, implying that PsPIP2-1 expression may have increased to compensate for AQPs blocked by mercury. ABA usually increased Lpr, but changes in PsPIP2-1 were variable and the direction of the response was strongly dependent on the dose of ABA that was applied. Overall, the coincident rhythms in Lpr and PIP2 expression and response to AQP blockage are consistent with the hypothesis that Lpr changes are mediated, at least in part, by changes in PsPIP2-1 expression. Inconsistencies with ABA data may have been due to more complex interactions of ABA with AQP channels.