We examined the effects of surgical and gastric transmitter implantation techniques on the growth, general physiology and behavior of 230 subyearling chinook salmon (Oncorhynchus tshawytscha, Walbaum) (100 mm-154 mm fork length). The transmitter weighed 1.3 g in air (0.9 g in water) and comprised, on average, 6% of the body weight of the fish (in air). Individuals were randomly assigned to an experimental group (control, surgical or gastric) and a sampling period (day 5 or day 21). Relative growth rate was expressed as% body weight gained/day. General condition was assessed by necropsy. Physiological response variables included hematocrit, leucocrit and plasma protein concentration. The mean relative growth rates of control, surgical and gastric fish were not significantly different at day 5. By day 21, the gastric group had a significantly lower relative growth rate (1.3%) as compared to the surgical group (1.8%) and the control group (1.9%) (P = 0.0001). Mean hematocrit values were significantly lower in the surgical (41.8%) and gastric (42.2%) groups as compared to controls (47.3%) at day 5 (P = 0.01), but all were within normal range for salmonids. No significant differences in hematocrit values were detected at day 21. Leucocrit values for all groups were ??? 1% in 99% of the fish. Both tagged groups had significantly lower mean plasma protein levels as compared to controls at day 5 (P = 0.001) and day 21 (P = 0.0001). At day 21 the gastric group (64.4 g 100 m1-1) had significantly lower mean plasma protein levels than the surgical group (68.8 g 100 ml-1) (P = 0.0001). Necropsies showed decreasing condition of gastrically tagged fish over time, and increasing condition of surgical fish. Paired releases of surgically and gastrically implanted yearling chinook salmon in the lower Columbia River in spring, 1996 revealed few significant differences in migration behavior through two reservoirs. We conclude that gastrically implanted fish show decreased growth and condition over a 21 d period. We recommend a surgical implantation method for long-term studies of juvenile salmonids, however, gastric implantation may be suitable for short-term studies.