Fast axoplasmic transport was examined in experimental nerve ligation and adriamycin-induced neuronal perikaryon lesions. Animals were sacrificed at precisely 3 and 4 h following the injection of [3H]leucine. In single ligated nerves, the 3 and 4 h down-flow rates, which include the time for neuronal processing of labelled material, were 379.2 +/- 23.51 and 370.8 +/- 26.93 mm/day, respectively, compared to the calculated rate of fast axoplasmic transport of 345.6 +/- 25.22 mm/day. In double-ligated nerves the down-flow rates were 364.8 +/- 40.32 and 342 +/- 32.2 mm/day, respectively, while the fast axoplasmic transport rate was 273.6 +/- 36.16 mm/day. In contrast, in adriamycin toxicity the down-flow rates were 363.9 +/- 21.54 and 378 +/- 14.69 mm/day with a normal calculated rate of fast axoplasmic transport of 420 +/- 18 mm/day. These studies illustrate that evaluation of two (or more) different time intervals can be used to assess the site of impaired delivery of labelled materials to the peripheral portions of the nerve. Furthermore, a single time interval examination of transport should not be equated with axoplasmic transport especially under pathological conditions.