Local cerebral blood flow (LCBF) in the conscious rat was estimated with one of 3 radiotracers -14C-antipyrine, 14C-iodoantipyrine or 3H-nicotine. A tracer was infused intravenously at a constant rate and blood concentration was followed until the animal was killed by decapitation. Tracer concentration was then measured in each of 14 brain regions. The Kety-Schmidt analysis was applied to the data with UCantipyrine and "C-iodoantipyrine. The results confirmed findings of Sakurada et al. (1978) that 14C-iodoantipyrine provides LCBF's that are twice those obtained with uC-antipyrine and that approximate LCBF's found with an inert gas. LCBF was calculated from the 3H-nicotine data by assuming complete extraction of tracer from blood by brain. This assumption was approximated for infusion times of 50 sec or less, when LCBF's derived with 3H-nicotine generally did not differ significantly from LCBF's obtained with "C-iodoantipyrine. Fifty-sec 3H-nicotine-derived flows for the pineal and pituitary glands were, respectively, 2.59 ± 0.36 (SEM) cm3 g ' min1 and 1.28 ± 0.08 cm3 g l min 1. LCBF's calculated for 70 sec to 240 sec of 3H-nicotine infusion were lower than 50-sec values due to back-diffusion, but nevertheless were as high as "C-antipyrine LCBF's due to the marked binding of tracer by brain tissue. The results supported the conclusion of Sakurada et al. (1978) that iodoantipyrine is the non-gaseous agent of choice for measuring LCBF precisely, but also showed that short infusion schedules with nicotine provided good estimates of LCBF.