Studies of algal biomass, ecosystem metabolism, nutrient uptake, and nitrification were conducted in Peruvian headwater streams in the Amazon River basin. These are the first assessments of stream functions in this region of the tropics and they also allow a preliminary assessment of functional responses to local watershed land use. Measures were made in two tributaries of the Rio Madre de Dios. Both had low gradient and primarily soft substrata but land use at one (Q. Concepción) was 2nd-growth forest while at the other (Q. Abejitas) it was pasture. The third stream (designated Q. Tambopata), a tributary of the Rio Tambopata, had higher gradient, primarily hard substrata, and a watershed of undisturbed forest. Algal biomass in Q. Tambopata was four to eight times greater than in the other streams. Algal primary productivity (P) in all streams was low due to shade, relatively low nutrient concentrations, and colored water at Q. Concepción, but was approximately twofold greater in the pasture stream than in the forested streams. Ecosystem respiration (R) in the pasture stream was approximately three and eight times greater than in Q. Concepción and Q. Tambopata, respectively. The ammonium uptake velocity (Vf) was nearly identical in all three streams, despite differences in land use and streambed characteristics. Nitrification Vf was relatively low in all streams, but was lowest in the pasture stream. Phosphate uptake (Vf) in the two forest streams was relatively low, while phosphate uptake in the pasture stream was among the highest of reported values, but perhaps the result of chemical precipitation as well as biological uptake. Carbohydrate (glucose and arabinose) uptake (Vf) was higher in the pasture stream than in the forest streams. Metabolism and nutrient uptake rates in the forested streams were comparable to or lower than reported values for temperate streams. Most rates of function in the pasture stream were elevated when compared to the forested streams.