Accurate measures of physiological and metabolic condition could provide more insight into how longevity genes and signalling pathways affect global metabolic activity and life span. The present study is essentially a methodological treatise in which we describe and evaluate a number of methods to assess changes of metabolic activity in ageing Caenorhabditis elegans. Oxygen consumption and CO(2) production rate assays, and measurement of the heat output by microcalorimetry are performed using live worms. For other assays, frozen (-75 degrees C) samples can be used. A lucigenin-mediated light production assay provides information on the metabolic capacity (scope for metabolic activity) of the worms just before freezing. Assaying ATP and ADP levels provides a measure of the instantly available energy. The XTT assay measures the activity of enzymes that can reduce XTT. Blue fluorescence emitted at 420-470 nm is a potentially useful biomarker of the rate of ageing. A protein quantification protocol for normalising all data for quantitative comparisons is presented. We illustrate how these methods can validate or disprove models of gene action inferred from molecular identification.