We conducted nitrate tracer experiments to determine nitrogen transformations of 9 low-order streams as part of the LINX II project.  These experiments investigated: 1) what controls nitrate uptake; 2) what controls denitrification rates; and 3) how important denitrification is to total nitrate loss in Puerto Rican streams. Short term additions of K¹⁵NO₃ and NaBr were performed on small streams with a gradient of human impacts. Several physical, chemical, and biological variables were also measured to determine controlling factors. Stream nitrate uptake length varied from 315 to 8480 m (median of 1200 m) and uptake length was mainly predicted by specific discharge and ecosystem respiration rate (r² = 0.71, p < 0.05). Other nitrate uptake parameters (V_f and U) were predicted by gross primary production and respiration, indicating strong biological control on nitrate uptake. Denitrification rates ranged from 0.01 to 2.20 ug N m^-2 s^-1 (median = 0.25) and the strongest predictors of denitrification were respiration and fine benthic organic matter (r² = 0.89, p < 0.05). Denitrification accounted for 1 to 97% of nitrate uptake with 5 of the 9 streams having denitrification accounting for greater than 35%, indicating that denitrification is a substantial sink for nitrate in tropical streams.