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(Source: C. Walker)
Young zebrafish of mixed sexes can be treated with testosterone to yield predominantly males. This is a useful procedure to ensure having enough males for breeding, particularly with lines that usually yield mostly females.


• 17-α-methyltestosterone
• MW 302.5
• Sigma #M-7252

Final concentration = 1.7 x 10-7 M

Testosterone Stock Solution:

150 mg testosterone in 50 ml Absolute Alcohol (very soluble)
Store at -20ºC

Testosterone Treatment Solution:

Add 10 μl of stock solution to 600 ml egg water slowly while stirring. Stir ~10 minutes.


1. Remove chorions from 24 hpf embryos
2. Add ~25 embryos to 50 ml fresh testosterone solution in a 250 ml beaker; add as little water as possible (i.e. 1 drop). Leave overnight.
3. The next day, add 50 ml fresh testosterone solution.
4. On each consecutive day, replace ½ of the testosterone solution with fresh testosterone solution.
5. Feed embryos normally.

6. Continue treatment until 15 days of development, moving fry from beakers to mouse cages at 9 days (see RAISING LARVAE IN A NURSERY, page 3.4).

7. At 15 days, remove most of the testosterone solution and add system water. Continue diluting during the regular daily cleaning.

Caution: Avoid contact with testosterone. Rinse well if you get it on your skin. Rinse glassware, etc. very well.

NOTE: Embryos need to be put in testosterone solution by 24 hpf. If treated later, some will develop as females.