Tesla and SpaceX stocks keep falling. Cathie Wood keeps buying the dip.

Shares of Tesla, the electric-vehicle maker, were down 1.4% in premarket trading at $304.82, while S&P 500 futures were down 0.1% and Dow Jones Industrial Average futures rose 0.3%. That price leaves shares down about 18% since the company reported weaker-than-expected second-quarter earnings on July 22.

Shares of rocket and artificial-intelligence company SpaceX declined 2.6% to $110.50 in premarket trading. That price leaves SpaceX shares about 18% below the stock's $135 initial public offering price.

The drops have been painful for investors. Of course, lower prices can also be an opportunity. That seems to be Wood's thinking. Her ARK Invest investment management firm bought more Tesla and SpaceX shares on Monday.

ARK purchased about 28,000 shares of Tesla worth roughly $9 million in two funds: the ARK Innovation ETF and ARK Autonomous Technology & Robotics ETF.

ARK also purchased about 125,000 shares of SpaceX worth roughly $14 million in four funds, the two that purchased Tesla shares as well as the ARK Space & Defense Innovation ETF and the ARK Next Generation Internet ETF.

ARK bought more SpaceX stock, but Tesla is still the largest position in ARK's flagship Innovation ETF, accounting for more than 9% of assets. SpaceX stock accounts for about half that percentage.

SpaceX, of course, is new, selling shares in a record-setting IPO on June 11. Shares rose 42% in the first three days of trading, but have fallen precipitously since then.

Tesla stock has struggled recently, too. Coming into Tuesday, Tesla stock has declined 31% this year and is off about 6% over the past 12 months, according to FactSet.

It will take time for investors to know how well ARK has timed the purchases. An upcoming catalyst for SpaceX stock will be the company's first quarterly earnings, due on Aug. 4. Catalysts for Tesla stock would include expansion of the company's robo-taxi service into more cities or the unveiling of the new version of Tesla's humanoid robot, Optimus.