A detailed study was carried out to obtain information on the relative rates of NH4+ and NO3−-N uptake and assimilation by perennial ryegrass (Lolium perenne L.), when grown in soil and supplied with different ratios of NH4+ and NO3−-N. Following two preliminary defoliations, plants were supplied with 15N-(differentially) labelled NH4+ and NO3−-N in three different ratios (20[ratio ]80, 50[ratio ]50 and 80[ratio ]20), followed by sequential destructive harvests of shoots and roots taken at four points during a 35-d regrowth period. When supplied with equimolar concentrations of NH4+ and NO3−-N, perennial ryegrass absorbed both forms of nitrogen at almost identical rates; and even when the two N forms were supplied in widely disproportionate concentrations, both forms appear to have been absorbed at equal rates, at least until the supply of the minor nitrogen component in each treatment was almost exhausted, i.e. by day 3 of regrowth. It is suggested that this matching of NH4+ and NO3−-N absorption rates by plants maximizes the advantages and minimizes the disadvantages associated with the exclusive use of either form of nitrogen, and thereby optimizes the potential for vegetative and reproductive regeneration. Fifteen days after the application of 15N-labelled fertilizer, shoot and root yields differed little between treatments. However, plants grown with NH4+ and NO3−-N at a ratio of 50[ratio ]50 had accumulated significantly more root (soluble) carbohydrate than those in the other two treatments. Consequently, when subjected to defoliation at day 15, these former plants were clearly in the best position to re-mobilize root carbohydrate and initiate shoot regeneration. However, further work is needed in order to determine whether or not this finding holds true for plants subjected to a multiple (bi- to tri-weekly) defoliation regime, e.g. in a grazing situation.