Fig. 2. Trimethylamine oxide (TMAO)-sensitivity of heat-induced change in net active cerebrospinal fluid-to-blood (CSF-to-blood) [¹⁴C]2,4-dichlorophenoxyacetic acid (2,4-D) fluxes across paired halves of shark IVth choroid plexus following 48 h in explant culture. Fluxes of 2,4-D were determined after tissues were exposed, ± 72 mmol l^–1 TMAO, to treatment as follows. Control, 7.5 h at 13.5°C; +5°C heat stress, 18.5°C for 6 h plus recovery at 13.5°C for 1.5 h; +10°C heat stress, 5 h at 13.5°C then 1 h at 23.5°C followed by recovery at 13.5°C for 1.5 h. Tabulated data show the mean unidirectional fluxes from which net data were derived. B to C, blood-to-CSF flux; C to B, CSF-to-blood flux. Data are means ± s.e.m. (N=6). ^*Significant effect of TMAO at P<0.05.