In the present study, hepatic p-nitrophenol glucuronidation was analyzed comparatively in virgin female, lactating mother and nonlactating mother rats (the last two groups 19-21 days post-partum). Enzyme assays were performed in native and activated microsomal suspensions. Activation was achieved either by including UDP-N-acetylglucosamine in the incubation mixtures or by preincubating native microsomes with optimal concentrations of Triton X-100 or palmitoyl-lysophosphatidylcholine. When UDP-N-acetylglucosamine was used as activator, enzyme activity increased in both lactating (about 80% increment) and nonlactating mothers (about 30% increment) as compared with virgin females. From an analysis of the degree of activation by Triton X-100 and palmitoyl-lysophosphatidylcholine, it can be inferred that the pregnancy-delivery event decreased the latency of UDP-glucuronosyltransferase activity that was detectable even 3 weeks post-partum, irrespective of whether suckling newborns were or were not kept with their mothers (lactating and nonlactating mothers, respectively). The estimation of apparent Vmax toward UDP-glucuronic acid in palmitoyl-lysophosphatidylcholine-activated microsomes, which allows an estimation of the amount of the enzyme, showed that lactation increased the number of catalytic units (about 40%). Hepatic UDP-glucuronic acid content was 70% higher in lactating rats than in other groups. The lipid composition and membrane fluidity (using 1,6-diphenyl-1,3,5-hexatriene as probe) were also analyzed in microsomes from all groups. A significant decrease in the unsaturation index that correlated with the rigidization of microsomal membranes was consistent with the changes in the degree of enzyme latency observed in lactating and nonlactating mothers. In conclusion, lactating rats exhibited enhanced p-nitrophenol UDP-glucuronosyltransferase activity as well as an increase in the hepatic content of UDP-glucuronic acid. These findings and the fact that lactation increased the liver to body weight ratio emphasize the role of the liver in the metabolism of planar phenolic derivatives in these circumstances.