We have investigated the multidimensionality of the free energy landscape accessible to a nucleic acid hairpin by measuring the relaxation kinetics in response to two very different perturbations of the folding/unfolding equilibrium, either a laser temperature-jump or ion-jump (from rapid mixing with counterions). The two sets of measurements carried out on DNA hairpins (4 or 5 base pairs in the stem and 21-nucleotide polythymine loop), using FRET between end labels. or fluorescence of 2-aminopurine in the stem as conformational probes, yield distinctly different relaxation kinetics in the temperature range 10-30 degrees C and salt range 100-500 mM NaCl, with rapid mixing exhibiting slower relaxation kinetics after an initial collapse of the chain within 8 mu s of the counterion mixing time. The discrepancy in the relaxation times increases with increasing temperatures, with rapid mixing times nearly 10-fold slower than T-jump times at 30 degrees C. These results rule out a simple two-state scenario with the folded and unfolded ensemble separated by a significant free energy barrier, even at temperatures close to the thermal melting temperature T-m. Instead, our results point to the scenario in which the conformational ensemble accessed after counterion condensation and collapse of the chain is distinctly different from the unfolded ensemble accessed with T-jump perturbation. Our data suggest that, even at temperatures in the vicinity of T-m, or higher, the relaxation kinetics obtained from the ion-jump measurements are dominated by the escape from the collapsed state accessed after counterion condensation.