Methods for estimating the carbon/energy costs of plant tissues from measurements of heat of combustion, ash content and organic nitrogen content were investigated. Calculations of maintenance cost (obtained by estimating the proximate composition of tissue from these measurements and applying published maintenance coefficients for various biochemical constituents) are valid only for tissues composed primarily of carbohydrate, protein and membrane lipid). Construction cost was calculated by estimating Glucose Equivalents (McDermitt & Loomis, 1981, Ann Bot 48:275) from these measurements and dividing by an efficiency factor. For most plant tissues, construction cost, as calculated from biosynthetic pathway analysis, may be approximated to within (+OR-)6% by using an efficiency of 0.89. Accounting for other growth-related costs, such as carbohydrate transport and mineral uptake, reduces this efficiency. Short cuts in methodology caused very little error in calculations of construction cost on a total dry weight basis. A survey of 29 diverse plant tissues indicated that tissue construction cost may cover a 2.5-fold range. Application of this method in carbon balance studies, allocation studies and other ecological studies was evaluated. Construction and maintenance costs for Diplacus flowers, calculated using the proposed methods, differed from estimates based on gas exchange and growth analysis. Differences in the assumptions underlying the two approaches may explain these dissimilarities. Although cost estimates differed, the overall pattern of flower carbon balance (ie. relative dependence on translocated carbon through various developmental stages) remained unchanged. The importance of measuring construction cost in biomass allocation studies was evaluated using two species of Piper. Accounting for differences in construction cost was necessary only when conclusions were based on relatively small allocation differences and when tissue costs were very different (eg. allocation between leaf lamina and petiole/midrib). The value of such a method in ecological studies was illustrated with a study on the relationship between leaf longevity and leaf cost. A survey of a successional series of Piper species, in which leaf longevity generally declined with succession, revealed that leaf tissue cost declined with succession. This unexpected result has resulted in refinements of theories regarding leaf longevity and cost.