Selected serum constituents were analyzed from 50 adult mallards (Anas platyrhynchos) of both sexes during several stages of reproduction: pre-egg laying, egg laying, incubating, molting, and postreproductive. Similar assays were conducted on sera from ducklings aged 5 to 58 days. Values for total protein (TPR), albumin (ALB), glucose (GLU), gamma-glutamyl transferase (GGT), calcium (CA), phosphorus (PHOS) and magnesium (MG) differed by sex. When all data were combined and analyzed for sex-related differences within each reproductive condition separately, all assays except lactate dehydrogenase (LD-L), cholinesterase (CHE), alanine aminotransferase (ALT), aspartate aminotransferase (AST), creatinine (CRN) and direct bilirubin (BIDI) differed between sexes during one or more reproductive periods. Each assay showed differences among the various reproductive conditions regardless of gender. The pattern of change differed between sexes. All assays except ALB, GLU, CA and MG showed age-related changes. Lipemia in the sample interfered with all chemistries except TPR, LD-L and CA. Results indicate that when using clinical chemistry as a diagnostic tool in the mallard, age and reproductive condition should be determined in order to compare the data to appropriate control values.