National Grid's adjusted profit before tax fell by 22% at actual exchange rates to £807 million in the six months to 30 September.
Excluding the impact of timing, adjusted profit before tax was up 1% to £916 million.
Operating and finance costs both increased over the year.
Adjusted operating profit fell from £1.4 billion to £1.3 billion, but excluding the impact of timing it rose by 4% to £1.37 billion.
UK Electricity Transmission adjusted operating profit excluding timing fell by 11%, but UK Gas Transmission, US Regulated and NG Ventures and other activities all registered gains of 25%, 19% and 4% respectively.
Capital investment increased by 7% to £2 billion in the first half.
Overall group performance is anticipated to remain in line with the expectations set out at the full year results announcement in May 2017.
John Pettigrew, chief executive, said: "We have delivered a solid financial performance in line with our expectations, made further progress to evolve our business and maintained a world class, safe and reliable service. Our focus on efficiency and innovation has reduced costs and generated increased savings for bill payers.
"We also invested a further £2bn in critical infrastructure during the period. Our improved performance in the US is encouraging, with this part of the business now representing an increasingly important part of our investment proposition. Having reshaped our portfolio in the UK, we continue to expect our electricity and gas transmission businesses to deliver high levels of performance.
"Our outlook for the year is unchanged, underpinned by our expectations for a stronger second half. We are focused on completing rate filings in the US, continuing proactive discussions with Ofgem ahead of the next regulatory settlement in 2021 and seeking new opportunities to grow the business and optimise our performance. We are confident that our strategy continues to create value for shareholders, delivering an attractive yield, and asset growth in the 5% to 7% range."