Let G = (V, E) be a simple graph, an L(2, 1)-labelling of G is assignment of labels from non-negative integers to the vertices of G such that adjacent vertices gets labels which at least differ by two and vertices which are at distance two from each other get different labels. The λ-number of G, denoted by λ(G) is the smallest positive integer such that G has a L(2, 1)labelling with all the labels are members of the set {0, 1, • • • , }. The zero-divisor graph denoted by Γ(R), of a finite commutative ring R with unity is a simple graph with vertices as non-zero zero divisors of R. Two vertices u and v are adjacent in Γ(R) if and only if uv = 0 in R. In this paper, we investigate L(2, 1)-labelling in zero-divisor graphs. We study the partite truncation, a graph operation that reduces a n-partite graph of higher order to a graph of lower order. We establish the relation between λ-numbers of two graphs. We make use of the operation partite truncation to contract the zero-divisor graph of a reduced ring to the zero-divisor graph of a Boolean ring. We compute the exact value of λ-numbers of zero-divisor graphs of some classes of local and mixed rings such as Z p n , Z p n × Z q m , and F q × Z p n .