Fig. 5. The effect of 30 µmol l^-1 ketanserin on the diurnal cycle of J_urea in R. marmoratus (Series VII). For each 6 h time interval following the first exposure to ketanserin (12:00-18:00 h), fresh 30 µmol l^-1 solutions were added to the chamber. White and black bars above the graph indicate periods of light and dark, respectively. Values are expressed as means + s.e.m. (N=8). Treatment values (white bars) labeled with an asterisk are significantly different from control values (black bars) at corresponding time intervals (P<0.05).