Ten nesting leatherback sea turtles on Trinidad were anaesthetised for electroretinogram (erg) measurements, using ketamine and medetomidine, reversed with atipamezole. They weighed 242 to 324 kg and were given initial doses of 3 to 8 mg/kg ketamine and 30 to 80 μg/kg medetomidine administered into an external jugular vein; six of the turtles received supplementary doses of 2·6 to 3·9 mg/kg ketamine combined with 0 to 39 μg/kg medetomidine. The lower doses were used initially to ensure against overdosage and reduce the chances of residual effects after the turtles returned to the water, but successful ergs called for step-wise dose increases to the required level of anaesthesia. Respiratory rate, heart rate, electrocardiogram, cloacal temperature, and venous blood gases were monitored, and blood was collected for plasma biochemistry. At the end of the erg procedure, atipamezole was administered at 150 to 420 μg/kg (five times the dose of medetomidine), half intramuscularly and half intravascularly. The turtles were monitored and prevented from re-entering the water until their behaviour was normal. No apparent mortalities or serious anaesthetic complications occurred. The observed within-season return nesting rate of the anaesthetised turtles was comparable with that of unanaesthetised turtles.