Abstract. Let λ be a nonderogatory eigenvalue of A ∈ C n×n of algebraic multiplicity m. The sensitivity of λ with respect to matrix perturbations of the form A A + Δ, Δ ∈ Δ, is measured by the structured condition number κ Δ (A, λ). Here Δ denotes the set of admissible perturbations. However, if Δ is not a vector space over C, then κ Δ (A, λ) provides only incomplete information about the mobility of λ under small perturbations from Δ. The full information is then given by the set K Δ (x, y) = {y * Δx; Δ ∈ Δ, Δ ≤ 1} ⊂ C that depends on Δ, a pair of normalized right and left eigenvectors x, y, and the norm · that measures the size of the perturbations. We always have κ Δ (A, λ) = max{|z| 1/m ; z ∈ K Δ (x, y)}. Furthermore, K Δ (x, y) determines the shape and growth of the Δ-structured pseudospectrum in a neighborhood of λ. In this paper we study the sets K Δ (x, y) and obtain methods for computing them. In doing so we obtain explicit formulae for structured eigenvalue condition numbers with respect to many important perturbation classes.